This article has a lot of interesting stuff, though it tends to be largely cheerleading LLVM's design decisions, rather than delving into the really interesting tradeoffs.

For example, some compilers use multiple levels of optimizer IR, for example Open64/Pathscale. This allows them to perform more high-level language-specific optimizations. LLVM forces front-ends to lower their code directly down to its fairly low level IR, thus throwing away some of this high-level information. It's possible to use LLVM's metadata system to preserve some of it, but this is subject to a variety of restrictions, and decorating a low-level IR with high-level decorations can be substantially less convenient than just using a higher-level IR. Clearly there are interesting tradeoffs here.

For another example, the author ribs Perl, Python, Ruby, and Java for not sharing backend code, and yet, years later, LLVM has still not been proven broadly viable for such languages, despite numerous efforts. Maybe someday it will be, but it's pretty clear that there are, yes, tradeoffs in play. Having everyone implement their own x86 backend has its downsides, but trying to get everyone to use one x86 backend has its downsides too.