In 2022, I think that the main split is between GPS bike computers or sports-specific watches vs. your existing smartphone or possibly non-sports specific smartwatch. Older non-GPS bike computers still exist, but I don't see them a lot. In comparison, your existing smartphone offers enough capability for many cyclists, especially leisure ones. It's true that using it as a bike computer drains the battery, but the target population is also not usually on a marathon ride. Further, basically everyone already has a smartphone. The mount costs a bit, maybe the same or less than a non-GPS computer. You get a lot more capability than a non-GPS computer, e.g. the ability to upload rides to social media or to Apple Health or similar.
For sensors, the older bike computers had an integrated speed sensor that was connected by wire or an older, non-ANT+ or Bluetooth protocol. (example product: the Cateye Padrone) Some had cadence sensors as well. GPS bike computers or watches will need to connect to an external sensor. Smartphones and non-sports smartwatches won't have ANT+ connections, and they may not natively support pairing with cycling sensors.
In my view, an exact cadence measurement isn't critical for experienced cyclists. It is possible to train yourself to maintain a high cadence. If the concern is knee health, then I believe that strength and mobility exercises plus correct bike fit are probably a bigger priority than raw cadence (unless you have typically maintained very low cadence).
Differentiation points in the GPS unit market
There is a market for basic, low-cost GPS units. Some units may have excluded compatibility with power meters. However, you need ANT+ or Bluetooth to be compatible with other cycling sensors. Therefore, I'm not sure how many units do this. The Lezyne Macro Easy (MSRP US$80) may be one of these; its description omits any mention of power meter compatibility. The Wahoo Elemnt Mini and the Garmin Edge 25 are other examples I remember.
Otherwise, lower-cost GPS units may be less refined than the higher-end ones. They may use cheaper screens, and probably use a monochrome screen. Software does play a major role in keeping the computer up to date with other apps and in refining the user experience. The more basic GPS units may have less refined software. Bryton (a Taiwanese company) and Lezyne (based in the US) are two example companies who offer a number of lower-cost GPSes. The original question includes a few manufacturers based in mainland China, and they are probably playing in this market.
Some higher-end GPS units are explicitly aimed at providing better navigation functionality. Examples include the Garmin Edge 1030 and Wahoo Elemnt Roam. Higher-end units are also more likely to use a color screen, which can help with visibility and reading the map, if you're displaying a map. These two examples also have relatively large screens. The second version of the Roam offers an enhanced GPS chip that has better resolution, which can be effective in remote areas, and I would bet many higher end Garmins do the same.
Regardless of tier, what would differentiate GPS units from different manufacturers? Most likely build quality and usability. Note that usability includes the software interface as well as how well the manufacturer maintains and updates the software. Garmin may have a past reputation for software issues. Wahoo's units are generally regarded as more user-friendly than Garmin, although Wahoo's mapping functionality lags Garmin's.
I don't see that there is a market for cycling computers that lack GPS but have ANT+ or Bluetooth connectivity. By the time you add those chips, I assume that adding a GPS chip would not cost much more. I recently checked the Bontrager link, and the RIDEtime Elite is actually an example of this type of computer. It was listed at US$75 MSRP. However, again, this type of computer competes with your existing smartphone, so I'm not sure how much demand there is for units like this.
Considering sports watches
Sports-specific smart watches may make sense if you do a lot of activities other than cycling. For example, perhaps you hit the gym in the winter, or you hike, swim, canoe, etc all round the year. A sports watch may make sense, as it will have protocols for specific sports. Many dedicated ones can pair with cycling sensors. You can often get bike mounts for them, although if you use them you'll lose function from the on-board optical heart rate sensor.
These are distinct from general purpose smart watches like the Apple Watch. In particular, the Apple Watch can't natively pair to cycling sensors (and it doesn't have ANT+ anyway, although some apps may let it pair via Bluetooth to other sensors), nor can it natively transmit its heart rate data to a head unit. Nevertheless, a watch like this may suffice for many leisure cyclists.
It's worth discussing sensors briefly. First, a GPS head unit can measure your speed with the GPS chip. While this is less accurate than a bike-mounted speed sensor, it suffices for most people.
A number of companies offer separate speed, cadence, or combined speed and cadence sensors. Power meters measure cadence, as this is required to calculate power. Thus, you basically get a cadence sensor for free if you use a power meter. Heart rate monitors are another common sensor type. Chest straps measure electrical signals and are regarded as more accurate than the alternative, which uses optical sensors and is usually placed on the wrist or arm.
For general interest, there are a number of less common sensors:
- Core body temperature sensors
- Blood glucose sensors
- Muscle oxygenation sensors, e.g. Humon
- Tire pressure sensors, e.g. Quarq Tyrewiz
- Possibly you can link a smartwatch and its optical heart rate monitor to the head unit, but I haven't investigated
- Lights: Not a sensor, but smart lights link to your head unit and can be controlled from there. Garmin may be the only major manufacturer that has implemented this protocol.
- Rear-facing radar: the Garmin Varia is a radar linked to a light that warns the user of incoming rear traffic and transmits the signal to a head unit. Garmin originated this, but many computer manufacturers implement a protocol for the radar. Additionally, other companies have started manufacturing radar units using the same (open source) protocol.
- Cameras, e.g. GoPro units.
- Electronic shifting systems can be linked to the head unit to display gearing or battery information. Older Shimano units require a separate transmitter.
In the past, e.g. the early 2000s and prior, I think the main differentiation point for cycling computers was wireless vs. not wireless, plus some units had cadence sensors. The Shimano Flight Deck and Campagnolo Ergobrain would display what gear you were in and were an early attempt at premium differentiation, although these never really caught on.
I can't remember exactly when GPS units were first introduced, but it was probably mid to late 2000s, and probably by Garmin. They were perceived as very expensive. Power meters began to be introduced around this time frame. I believe that companies used proprietary computers, e.g. the SRM PC7 and earlier, and I think PowerTap had a proprietary computer as well. Later on, the ANT+ standard became accepted throughout the industry, and computers started adding Bluetooth compatibility as well. This would have been a differentiation point at the time.
Before GPSes, some people trained with heart rate. I remember that Polar was the main player in the space, or one of the main players. The HRM readout had to be displayed on a separate display from your bike computer. I remember my first HRM having a watch that you could mount to the handlebars.
Smart phones were only coming into widespread use around this time, and I don't recall that it was common to use phone mounts.
A side note about non-GPS units: some ultra-distance races in remote areas may require you to have a non-GPS unit on your bike. These types of races will often hand out paper cue sheets at checkpoints, and you must arrive at a checkpoint by a certain time to be allowed to proceed on the course. In my subjective opinion, some organizers give off an attitude that seems snobby and retro-grouchy to outsiders. However, for these rides, you may not have reliable GPS reception. You will need an accurate distance measurement to use the cue sheets. In addition, many of these events want to conceal the route from the riders so that some of them don't arrange for external logistical support; this levels the playing field among all riders. Last, there's always the possibility that you a GPS unit could run out of power.