A lot of effort goes into making sure allocations are avoided though. You also have DOT's High Performance C# which is a blazing fast subset of C# (Burst Compiler). I like the idea of using a language for everything, then restrict to a subset where performance is critical. You don't have that wall between scripts and (C++) engine code. And as mentioned above, less glue code is appreciated.
Garbage collection probably. Some games have noticeable stutter. But I assume there are ways around that, similar to using Java in HFT.