There are two things the Ace Pro 2 has that the others don't. The first is the Xplorer Grip, and the second is the Leica Summarit lens. No, it's not an M-mount Leica, but it's still a very good lens, better for stills in my experience than other action cams.
It's a 13mm lens (35mm equivalent) with an f2.6 aperture, which means you can get some extremely wide-angle shots. I find it's fun to play with leading lines in creative ways when I shoot with it. Despite the relatively wide aperture, there's not much depth of field here—pretty much everything is always in focus. Again, this is an action camera. There are other lenses you can add: a macro, an ultrawide angle, and a cinema lens. The best of the bunch from a photography standpoint is the macro, but I haven't found myself reaching for any of these lenses very often when shooting photos. (I use the ultrawide all the time for video.)
While the Ace Pro 2 is no full-frame-camera replacement, it does have its own benefits—like durability, waterproofness, and compact size—that make it capable for shots you wouldn't want to risk with a $2,500-plus mirrorless camera. Even when I bring my "real" camera, I still throw an action camera in my bag to help get shots I might not without it. It's also just a fun camera to use.
A great example of this is handing a camera to someone to take a photo of you or giving it to my wife or kids to shoot with. Handing them an SLR-style camera in full manual mode requires a minute of directing and help. In contrast, I sent my wife and daughter out to a winter survival camp they did earlier this year with the Ace Pro 2 and didn't explain a thing about it. The case has one big button and one dial. I put the camera in photo mode, auto exposure, and stuck some gaffers tape over the dial so the setting wouldn't be accidentally changed, and sent them on their way. They came home with some great photos.
Another thing you can do with the Ace Pro 2 that I would not do with my mirrorless rig is take it out on the water. This may be a lifestyle thing, but I try to spend as much time as I can in and around water. My Sony A7R II isn't even weather-sealed. Neither are any of the lenses I use. A waterproof housing for that camera is 3X the price of the Ace Pro 2. I much prefer to slap a float on the Ace Pro 2 and jump on the paddleboard. This is the one place the grip was a little disappointing; the grip on the Ace Pro 2 is no longer waterproof.
The Best Photography Settings
Photograph: Scott Gilbertson
If you decide to pick up the Xplorer Grip, here's how to take better photos with the Ace Pro 2.