[jacques_chester]

Producing a compiler that is robust on a single platform is relatively straightforward. You just bash away for a while; with wide usage the corner cases will be exposed and you can converge to a finished system.

Add multiple platforms, and it gets more difficult, because each platform has quirks and peculiarities you don't know about until you get a bug report.

Add optimisation of languages that are difficult to automatically reason about, and it gets harder again.

Multiply the optimisation difficulties by multi-platform difficulties and yes: it is hard.