In certain industries, specially where human lives are at stake you can only use certified compilers, where the kind of code they generate is validated against specific safety standards.

For example,

https://www.iar.com/iar-embedded-workbench/certified-tools-f...

As a programmer using the compiler, how does the IAR compiler quality feel in practice compared to GCC or clang?

I never used IAR, given my focus on writing safe software I try to be up to date to these areas, and ways to somehow write secure software in C and C++. like MISRA and Frama-C.

Maybe someone else can jump in.

However, I can tell you that in general proprietary compilers for embedded devices make Visual C++ feel state of the art, let alone when compared with gcc or clang.

Many of them are stuck in partial support for C99 or C++98 (e.g. TI), with no clear roadmap of future improvements.