Certainly using Illya isn’t too complicated. Though she’s fine at what she’s there for. Hit one enemy really hard, survive until her NP is charged back up, and then hit them again. One more burst damage servant among many that are the same in that respect. Only really one thing she’s suited for, but she’s got the kit and noble phantasm for it. If you have others for that kind of role then nothing too much to say for her, but that’s the case for almost any servant. There are multiple options to deal with any given problem.
Luckily isn’t too hard to make her work or even to deal with the demerits on her NP. All the more so now with command codes and an ever increasing roster of helpful servants to throw out there with her. Not going to spam her NP easily, but that’s fine since it’s not how she’s set up to work anyways.
On the Miyu topic, yeah there’s a solid and expected benefit there. Ideally of course you’d want to use Illya’s skill on herself to avoid NP demerits, but there is the option to bolster Miyu if she’s low on health. In general though it’s a solid pair. Illya needs to get her NP charged to do her damage so that’s fine and more stars can mean more arts crits which helps towards that also. And considering Illya is there to hit, survive, and hit again, keeping her health up is beneficial. But of course there’s a lot of solid options with Illya to keep her alive, hitting harder, and dealing with Assassins. Miyu just adds one more piece to the pile.