Always amazed at how many folks claim GURPS is complex as they sit down with 10+ books for their next PF/D&D game so they can sort through 50+ races/species/ancestries, 100+ classes/subclasses , several hundred feats, and skills that may break down into 3 or more sub skills. But GURPS is complex.....
I tend to view GURPS less as a game and more as a toolkit to build a game with. With most games, even a complicated game like Pathfinder, you're getting a complete game in the package. The GM doesn't have to create the player character classes, species, spells, equipment, skills, etc., etc. because that work has already been completed. With GURPS, you're getting a toolkit to build a game with. You want elves? Well, better create one. (I know they have examples of common species in various sourcebooks.) You want wizards? Better decide how spells work and then create them. When it comes to GURPS, the GM, and I'd argue the players, have to put in a little more work upfront to get the game going.
The last time I used GURPS was circa 2005, 4th edition, and I was running a Delta Green campaign. As far as world building goes, it was relatively easy because it's set in the modern era, all the PCs are human, and at the beginning they don't have access to supernatural abilities. I gave the players somewhere between 150-200 points to build their characters with instructions to avoid any supernatural advantages, disadvantages, and powers. And of course I offered guidance such as suggesting Advantages like Legal Enforcement Powers for the PCs who were special agents with the FBI and the ATF. I really only had one player who was difficult, but I chalk that up to the player rather than any problem with GURPS itself.
The biggest problem my players ran into were the sheer number of skills available. It just felt like a lot of work on their part to pore over the list and decide which ones might be of use in the campaign. This is when one of the players laughed upon discovering there was a Dropping skill. I did have one problem player whose opinion of what a "normal" human differed greatly from mine. He kept trying to buy psychic powers and silly advantages like extra arm strength and I constantly had to tell him no. This was more of a problem with the player than it was with GURPS I think.
The drawbacks of GURPS came for me as a GM when I had to decide how magic works and to stat out creatures. What abilities should a ghoul have? How about a wizard? A byakhee? It was a bit more work than I cared to put into the game. As a GM, I'd rather concentrate on interesting plots and encounters rather than building creatures, NPCs, spells, etc., etc.