My game was complete, but the garbage collector would randomly crash, every hour or so. I spent nearly two weeks reading through incomprehensible C++ code, and completely failed to fix the problem myself.
Hugh, the creator of NekoNME, graciously helped me after I sent in a deterministically crashing test case, and that's the only reason the game was shipped at all. I consider myself a competent developer, but utterly incapable of debugging someone elses GC.
I also had to make a whole bunch of smaller fixes (eg adding in app purchases, fixing leaks etc), but those things I expected.
If you're evaluating an engine for use, there should either be a flawless game more complex than what you're trying to accomplish, or you should be ready, willing and able to traverse the full stack fixing whatever comes up.
(Note: Haxe is battle tested now.)