No, C doesn't actually handle the _complexity_ of writing assembly. It abstracts and automates a lot of the repetitive work of doing register allocation etc -- sure. But these are very local issues -- I think it's fair to say that the complexity of a C program isn't really much lower than the equivalent program hand-coded in assembler.

I'm not sure that LLVM would be the first consideration for complex, flexible, efficient? It's quite certainly not fast, in particular linking isn't. I'm not sure about Chromium, it would be interesting to look at some of the more interesting components like V8, rendering engine, OS interfacing, the multimedia stack... and how they're actually written. I'd suspect the code isn't slinging shared_ptr's and unique_ptrs and lambdas and is keeping use of templates minimal.

I would have thought of the Linux kernel first and foremost. It's a truly massive architecture, built by a huge number of developers in a distributed fashion, with many intricate and highly optimized parts, impressive concurrency, scaling from very small machines to the biggest machines on the planet.