I play a Unity game; Hearthstone. The number of engine-related bugs shocks me. My favorite is how the game completely locks up at the start of every match. Sounds stop playing, the UI freezes... it looks like the game has crashed. (I believe it's writing a ton of text to a log file, which is unfortunately part of their public API now because of deck trackers that read those files. Maybe it's also reading assets; I've never profiled it.) What amazes me is how consistent the behavior is across all platforms. It locks up on Windows, and it locks up on my iPad. Write once, run anywhere!

Of course, I don't think it would be worth writing your own game engine to work around that one bug, but at some point, one wonders how many oddities it takes to justify a rewrite. People still play the game despite this one bug that's existed (I hear) since launch, so I guess it was a good cost/benefit decision to use Unity. But for a game studio that prides themselves on polish, that particular bug makes me die a little inside every time it happens.