It's like having a whole nother Warlocky but not Warlock class to worry about. It's got mini spells and mini feats like almost Invocations things.
I don't think this is a real issue.
As a DM, you likely have 3-7 characters in the group. Of those, maybe half are usually going to be playing significantly complicated/tricky classes (by D&D 5E standards, so not very complicated by more objective standards). The Illrigger doesn't add to that load, it's probably replacing one of the other complicated/tricky classes.
Is it worse than having a Warlock with a funky setup or, I dunno, pretty much any Paladin, or god help us, a Bard?
No. Not really.
Along with it being soft coded as evil so you could feel the need to incorporate recurring nonviolent evil dealing in your story.
I can see many DMs not wanting to deal with it.
Again, this is a bit silly. Warlocks are soft-coded as evil. Rogues are arguably soft-coded as evil, especially with certain subclasses. Shadow Monks are soft-coded as evil. Many things in D&D are "soft-coded" as stuff - doesn't mean you have to act on it. You haven't explained why anyone would feel the need to "incorporate recurring nonviolent evil dealing" in your story? What is an example of what you mean?
I'm not like a huge booster for Illriggers - they have a totally dumb name for starters, sorry Matt, just a total miss with that one! But of all the 3PP classes I've seen, they're one of the least obnoxious.
They're certainly leaps and bounds better-designed than say, the godawful Blood Hunter, which, depending various choices, is somewhere between "literally the worst class in 5E" and "grotesquely overpowered", and just is absolutely top-to-bottom a total mess design-wise (embarrassing honestly).
In fact, let me go further - Illriggers are better designed than Artificers, period. Just in all ways. They're better balanced. They're less of a headache. They fit into a party better. So if you as a DM can handle an Artificer, you can handle an Illrigger. Except maybe for the name. Jesus I dunno if I could keep saying that name for an entire campaign. We'd probably need to come up with a replacement.