A handheld, image stabilized still shot from "Singing in the Rain."
It's hard to make definitive statements about the effectiveness of image stabilization across brands, models and different sizes of image sensors but...someone asked and I thought I'd give it a try. I haven't shot with all that many cameras so I'll focus on the ones I know of from experience.
The current top dog of image stabilization, with utterly miraculous performance, is the latest Olympus EM-1 mark 2. Nothing else can touch it. Unless you are a ten cup per day coffee drinker you can pretty much ditch your tripod. This should not surprise anyone who had previously shot the EM-5 mark 2. I owned several of them and they were steady enough to use hand held when shooting video (see the Cantine video for end to end Olympus EM-5 mark 2 samples --- in motion).
The one addition that makes the EM-1.2 the best of the best is the ability of that camera system to use both the lens I.S. and the in body I.S. together. At best the system delivers 6.5 stops of stabilization. If not for the god awful menu system I might have bought that camera instead of the GH5.
The GH5 also has a dual I.S. feature and it works very, very well with a limited number of lenses from Panasonic. Using just the in body stabilization is very, very good as well. I've been using it with several Olympus zooms and several current Panasonic primes and find it to be in the same class, overall, as the Olympus EM-5.2.
The stabilization in both the RX10ii + iii, as well as the Panasonic FZ2500, is equally good. It's the latest five axis variety and since the lenses are built specifically for the body/sensor system I believe they are able to optimize their I.S. to good effect.
As we move up the format size in cameras that use in-body image stabilization we get less and less overall effectiveness from their systems. The Sony a6500 is at least a stop or a stop and a half behind the m4:3rds cameras and the Sony A7ii and A7Rii are at least a stop worse off than the a6500. And most of the bigger cameras have yet to incorporate competitive dual body+lens I.S. systems.
All bets are off when it comes to comparing systems that only use lens stabilization. For one thing not every lens you want to use is stabilized. But when done well the lens stabilization can be quite good.
When I worked with Nikon I bit the bullet and bought an 18-200mm lens even though I knew it was not a spectacular performer optically. I bought it because there were situations when I really needed stabilization and would not have time to use lights or a tripod. That lens delivered a stable shooting platform and was an eye opener. Rock steady shots but humdrum optical performance. A trade off.
Not so with the newest Panasonic and Olympus flagship systems. You get rock solid stability and great optical performance from their Pro series lenses. My least satisfying image stabilization experiences have been with cameras like the Sony a850 and a900 full frame cameras. They gave, at most two stops of stabilization (which is nothing to sneeze at compared to ancient times) but with some lenses they seemed to perform with less enthusiasm, delivering maybe a stop different at longer focal lengths.
My (unproven) assumption is that it's all about overall sensor size, acceleration and physics. The smaller the mass you have to move and the smaller the distance you have to move it the higher the performance is going to be and the more accurate the corrections will be.
All things being equal (but they never are) logic would suggest that one inch sensors should be the easiest to imbue with the highest stability performance followed by the m4:3rds and then the APS-C sensors and finally, in next to last place, the full frame sensor cameras. In last place would certainly be the medium format cameras whose only advantage is sensor size and, when dealing with camera motion whose biggest disadvantage is.... sensor size.
1200+ images shot handheld with the GH5 and some Olympus Pro lenses last night show me that the system is highly competent and workable for me. If I wanted the best offered anywhere I'd be putting those Olympus lenses on the front of an EM-1.2 camera body and never look back. But I.S. is only one factor among many in the pursuit of good photography. Compromises abound. That's what makes it all so interesting.