My main problem with it at the time were that the groups I was most heavily involved with that used GURPS, used it to run amazingly boring campaigns.
I could never quite figure out if it was the group / GMs who ran GURPS or the system itself, but my experience was identical. It's the only system where I broke my own rule of not looking on my phone / surfing the internet while playing, because the GMs could never figure out what the hell they were actually doing.
It's as if the chore of running GURPS somehow takes away the ability to actually create situations/scenarios the players can actually invest in / care about.
No sense of pacing. No drama. Zero stakes. Nothing interactive or interesting or useful about the settings. So many useless "Roll an INT check to spot something"; "Roll a horsemanship check to approach the stables . . . oh wait, no horsemanship? Okay, roll INT -5 as a default instead." Bland, boring, repetitive, "rowboat world" interactions (as
@Celebrim would call it), interspersed with restless players also getting bored and initiating combat at the drop of the hat just to have something to do . . . only to have each round of combat take multiple minutes each to go through the exercise.
Calculate facing. Calculate actions available to do in 1 second. Calculate attack roll. Roll active defense. Roll passive defense. Roll damage. Roll hit location. Roll damage penetration through armor. Calculate actual damage. Roll health + knockdown + shock + consciousness checks.
15 minutes later, someone else gets a turn.
* NNNNNgggggghhhh Snoooooore * Oh, I'm sorry, is it my turn? Did something interesting happen?
I had already abandoned D&D proper when I first starting playing GURPS back in 2009, played in 3 different short lived campaigns up through maybe 2019. But I knew right away, whatever the problems D&D had in play, GURPS wasn't the solution to whatever it was I was looking for.