There's a lot to be said about how GURPS is clunky - and it is, even when you're not digging into the insane optional rules - but I think its biggest flaw, by far, was that it had very little appreciation for genre. GURPS was built on the notion that a single system could tell any story, but what that meant in practice was that a single system could tell a story with different trappings. GURPS was set dressing applied to a core system that ultimately imposes certain genre constraints on anything you tried to run with it. This is true of all systems, but GURPS tries to act like it isn't.

That's not to say that it's universally bad. A lot of the settings for it do fit well in the mold it offers. Alpha Centauri is fantastic in GURPS, and it's a wonderful system for the huge line of historical settings it offers, if you're looking for a more grounded, simulationist take. But there are also plenty that don't. Illuminati University is brilliant but has no business being a GURPS setting. That game desperately wants to be PbtA. Ditto Discworld. Star Trek makes no sense; that game probably wants to be running on something like Cortex, the relationship engine would work beautifully with Trek's tone and themes.

God help you if you actually think the WoD conversions are a good idea. And who in their right mind ever thought it was the right system for Hellboy?

No system can ever be universal, because roleplaying games are stories, not historical re-enactments. That matters.

My perspective might be a bit more biased on that first point since I only got into the system in the late 00's.

But I disagree with the second point. I almost exclusively GM for my group, and I'd hardly consider anything I had to do to run the game a "slog". There's about as much prep and rules checking as any other system. Maybe a bit more front loading if you want to make a custom race available, but that didn't take too much more time that doing the same in Pathfinder 1e or DnD 3.5 did.

I mean, character creation takes longer because you have total control over everything, but that's basically a mini-game in itself. Honestly I find it much easier to get up to speed running a GURPS game than D&D. The books are better organized, the mechanics are more consistent, descriptions are more clear, everything is better cross referenced, character creation programs include page numbers with your traits for easy reference.

All you really need to know as a GM is how the success roll works, and you can basically look up or wing everything else as you need it. Maybe if you try to learn how everything works, it can seem like "inelegant bullshit", but not even the books recommended you do that. GURPS works best when you treat it as modular, adding things in as your players or world demand. All the mechanics you definitely need fit easily on a single sheet of paper.