If one wants to buy/build an ebike that is the most future proof as it can get, I would recommend to stay away of the following:
Popular manufacturers of those systems are Shimano, Bosch, Brose, Yamaha , Panasonic, etc. Their development mentality is to control every variable of the systems (hardware, software, user interface, protocol, replacement parts availability, etc) much like Apple does in the tech industry. The advantage is to have a very optimize system, but your are completely tied up when it comes to fixing.
They also do not sell small replacement parts like bearings and tell you that your whole mid-drive motor needs to be replaced when the bearings are worn-out (which will happen, they are bearings!).
For some of them like Bosch, you also need to attend their certification course (only available to authorized bike shops) to even have access to their diagnostic probe and software.
Most of the mid-drive motors need a frame with specific mount points. Those mounts points are not standardized so if you buy an ebike with a Shimano system and want to swap it later for a Bosch one, you can't. Even within the same brand, the mount points can change between generations.
Some mid-drive motors can be installed on a regular bottom bracket shell (e.g. Bafang), so those would be to prioritize if one wants to go the mid-drive way.
Custom-shaped battery packs
Even if an ebike uses a more open ebike system (i.e. mostly Chinese brands), the battery pack enclosure could be custom made for the specific bike model so it can, for example, fit inside the downtube (semi-integrated or integrated). If the pack goes bad and you need a replacement, you are totally dependant on the bike manufacturer.
One can of course go to a company that can replace the Li-Ion cells, but if the BMS is fried, things get harder. It is better to prioritize ebikes with the battery mounted on a rear rack or using the water bottle cage holes.
Even if an ebike uses open components, it does not mean you can buy them. Some of those components are OEM parts, which means the parts manufacturer (e.g. Bafang) will only sell them to bike manufacturers that get them in large quantity and not to bike shops. This make finding a replacement very hard and will often lead to replacing the controller/lcd/wiring combo (and something even the torque sensor which output signals are less standardized than the throttle for example).
Some ebike manufacturers will re-brand third-party ebike parts to put their name on. For example, we can recognize that RadPower uses the King-Meter SW-LCD. The thing is, we do not know if they ask King-Meter to put a custom firmware inside or to change the connector type making the original LCD incompatible. We would then need to rely on the bike manufacturer to get a replacement instead of the part manufacturer, which is a less reliable source.