For .NET, there are two disadvantages that I can see.
1) People assume that the GC knows best, but that's not always the case. If you make certain types of allocations, you can cause yourself to experience some really nasty program deaths without direct invokation of the GC.
2) Objects larger than 85k go onto the LOH, or Large Object Heap. That heap is currently NEVER compacted, so again, your program can experience out-of-memory exceptions when really the LOH is not compacted enough for you to make another allocation.
Both of these bugs are shown in code that I posted in this question:
How do I get .NET to garbage collect aggressively?
