... is, I think, "simple control".

Of course, that's necessarily vague - what, exactly, is "simple"?!

I guess "simple" is "not so complex that the architecture's inherent limitations become significant obstacles" - in other words, where the architecture is not a "misfit".

Again, of course, "significant" is vague.

As I always say: if you're fighting the architecture, then it's the wrong architecture!

In the case of the 8051, things like banking (usually) constitute "fighting the architecture".

In the case of an ARM, if the architecture is "overcomplicated" or "too expensive" for the application, then the ARM is a misfit in that application (and an 8051 may be a far better choice).