True, not all have them (though a lot of 16mm and AFAIK almost all 35mm do). While I wouldn't claim that Holga-style still rotary shutters are absolutely stable, they're surprisingly far into "not bad" territory. I had a Holga shutter tested, and while it certainly wasn't as accurate as an electromagnetic shutter, I was surprised by its consistency (I don't remember the exact tolerance as it was a few years ago, but it was around the same as a spring-driven curtain shutter).
For a crappy shutter, the Holga's isn't bad. The hole is at the end of disc travel, where it's more likely to be at maximum and consistent speed. If you had an adjustable hole, you probably wouldn't be able to simply say that half the size will yield exactly half the exposure time because of acceleration, as you mentioned. But you can build a shutter tester to interface with a computer sound card (or build one with an
Arduino) very cheaply, and if the gap is continuously variable, it should be fairly trivial to figure out the correct spacings by trial and error.