Adrian might have mentioned an important drawback of using LLVM: active effort is required to keep up with it. Many an LLVM-based research project has gotten stuck on 2.9 or 3.2 at which point it starts becoming less and less relevant.

It's not that big of a deal, but active effort is required. The amount of effort depends on how many and which APIs your project uses; for a small/medium project perhaps a couple of hours every couple of weeks.