They are technically mergeable because they're text based, but they're not super easy to comprehend. I've found that .unity scene files seem to develop dramatic changes between check-ins that make it difficult to isolate problems (ie. giant blocks of text removed from one part of the file, then put in another part of the file untouched).
In general I feel like the best practice is to touch these .unity scenes as little as possible, and spend most of your time iterating on prefab files, which are easier to read.
Haven't tried git with unity so I dunno if there are any special case issues there.
They are pretty git friendly. There's instructions floating around for how to do it but it basically works as you'd expect once you get your .gitignore file set up properly.
The Editor loads assemblies dynamically as well so you can branch/checkout and the Editor will automatically recompile everything without a restart.
Only problem I've had is with scene objects but I think I recently fixed that once with a tweak to my .gitignore.
In general I feel like the best practice is to touch these .unity scenes as little as possible, and spend most of your time iterating on prefab files, which are easier to read.
Haven't tried git with unity so I dunno if there are any special case issues there.