The load limit is determined by the stand rather than the clamp in all but the cheapest setups. If you look at what bike shops use they tend to have great big chunks of steel with clamp arms bolted to those. But those clamps, even the really good ones, cost about the same as a cheap clamp with stand. So the question to ask right at the very start is whether you're better off buying the proper tool right at the start.
In both these photos (search for "bike mechanic" rather than "bicycle workstand") the key point IMO is that there's a nice solid bit of steel pipe holding the clamp up, and if you look around that pipe is normally bolted to the floor. Where I worked we used slightly thicker square tube with a clamp each side (two mechanics per pole).
(via Boston News)
That setup will be more expensive that a cheap portable stand, but a lot more effective. Find a local steel fabrication business, tell them what you want, say you're willing to wait until they have time to put one together, and it shouldn't cost too much. Most places will be able to make it from offcuts, and it's an easy welding job. What you want is thick wall mild steel square tube somewhere between 70mm and 120mm on a side and 5-10mm wall thickness, between 1.5 and 2m long. Weld that to a bit of plate about 400-600mm square and 8mm-15mm thick. The reason for the loose dimensions is that you are aiming for "what spare bits do they have", if you specify it exactly they will cut it for you and charge accordingly. So that post will cost between "some beer" and "$300 plus tax", depending.
I used to use a simple length of square tube with the clamp on it, and I'd put that in the vice on my workbench. But that was a 6" engineering vice that weighed 30kg, bolted to a 100kg steel framed workbench. If you have a bench like that it's a great way to get an easily moved out of the way stand.
(via Mσᶎ's awesome Paint skills)
A stand like that will hold an electric assist loadbike without problems once it's bolted to the floor. With a bigger plate you can avoid bolting it down, but it might fall over if you don't pay attention (and it's heavy enough that if it falls on your bike it will seriously damge the bike, or you). Lifting a heavy bike into the stand, on the other hand, will be a hassle. My local eBike shop uses powered hoists for that reason.
A cheap stand will be very flexible, and even if it doesn't actually collapse, it will feel as though it's about to even with a lightweight road bike in it. When you want to apply any real force to the bike you will need to brace the bike against that force as well as applying it. Which is a huge pain. That said, they do work for most minor repairs and adjustments.
But the stand is cheap and it folds away to nothing. Hopefully when you want it to fold, but for that price you can't be too fussy. What you're buying there is light and easy to fold, you're not paying for a usable workstand.
A better lightweight design is this style:
They support the bottom bracket rather than the top tube/seatpost, so there's less stand to flex. It's slightly more twiddly to get the bike in and out, but they're easier to work with in my experience. At least for upright bikes - it's unlikely that a recumbent or load bike will fit into them at all.
edit to add:
Finally, many "bike storage systems" actually work ok as workstands. The ones that suspend the bike from the roof using pulleys are very close to the power hoist that my LBS uses. Many of the "arms poking out from the wall" style have arms long enough that you can pull the bike out a bit and spin the pedals while the bike is on the storage unit. This style of leaning hanger is available pretty cheap if you can put up with less decorative versions:
With the "wall hook" style that hang the front wheel it's less convenient, but you can put a block under the bottom bracket to push the rear wheel away from the wall and adjust the gears that way. Reaching the handlebars when the bike is hung up is only easy if you're tall, though, so it's not always an option.
Many car racks can also be used this way.