Compared to its parent manga, Illegals has a much tighter focus.
The core cast is composed of a mere three members, as opposed to BnHA's massive cast herd. This is great for character development, with the main character and his teammates each getting significant character moments nearly every single chapter.
And even non-main-characters have it good-- The author doesn't treat them like setpieces to be moved around at his bidding, but respects their agency as individuals in a universe he's merely narrating.
Just like in BnHA, every superpower is unique and interesting, as are character designs.
Illegals' characters are perhaps the best part of the manga, and I rate
The monthly release schedule of Illegals lends itself to more episode chapters, with individually more compelling stories than any single BnHA chapter. Every month I'm excited to read the chapters because I know I'll get a self-contained experience that weekly manga can't quite match.
On the other hand, the monthly schedule detracts from Illegal's ability to significantly progress its story-wide arc, but luckily that's partially mitigated by how Illegals ties into BnHA, allowing information to be released in parallel.
I would rate Illegal's story as slightly worse than BnHA's story, but still very good on an objective level. 8/10.
I'll admit straight out that I'm not a good art critic-- I can say that the art looks good and that I personally enjoy it, but beyond that, not much.
If you liked the art in BnHA, the art here is basically identical. As a result of the smaller scale of Illegals (and presumably the smaller scale of its finances), it does lack a little of BnHA's "wow" factor. 7/10
Overall, I'm really enjoying Illegals, both as a sidestory to BnHA and on its own merits. In fact, I'd go so far as so say I enjoy it more than BnHA.
If you're a fan of street-level superhero stories, read it. If you're a fan of BnHA, read it. If you're willing to accept Pop*Step as your lord and savior, READ IT.
9/10 enjoyment, 9/10 overall.