I will say, it's a lot of work to make "real PC" like NPC's in 3.5e.
It depends on whether you are going for 'publication grade' NPCs or 'game grade' NPCs.
A lot of that IMO is burden DMs impose on themselves. You don't have to get it right. You just have to get it to where it works.
I run a semi-sandbox sort of game with a broad-narrow-broad sort of structure (with a willingness to flex if my players are proactive). By the definition of a Sandbox game, a large portion of what I prep I'm never even going to use. I've prepped to one degree or the other 3 dungeons - one of them to a finished state - that the players ended up bypassing. I can't afford to be wasting time with detailed stat blocks for NPCs that may or may not be even encountered and even if they are, may or may not utilize anything I write down. If I get it an NPC's stat block slightly wrong, I don't freaking care. It's not going to actually matter in game if in fact I used 6 to few or 6 too many skill points in the stat block, if I miss added an attack bonus by a couple of points, or I'm not squeezing every last drop out of my system mastery. It's really rare that that one NPC is really that critical anyway.
It's not like 1e stat blocks were really complete. They typically said something like "Eric Redbeard (F5)". That's not a complete stat block. How many hit points does he have? What is his str, dex, con, wis, int, or chr? What are his weapon proficiencies? What are his non-weapon proficiencies? How much wealth does he have? You were lucky if somewhere in the text block beneath that line it said something like, "Eric has a suit of plate mail, a shield +1, and a longsword +2." with the magic items bolded. No one worried about whether or not Eric was as powerful as a PC.
You know what, if it comes up and you need it, then you wing it. Your telling me you've been running 3.X for 10 years and you can't wing a CR 8 encounter and get it close enough? Who cares what spells the wizard has memorized; if it matters flip open a Player's Handbook and start casting likely spells. Jot a few likely defensive spells down for his spell list and get on with it.
You've been running 3.x for 10 years and you haven't generated a similar PC stat block in the past that's good enough for the situation?
Sure, it's nice to be able to refine a few critical NPCs in great detail. Just recently I had a goblin cook become interesting because I jotted "professional 3" down, and then decided to actually fill out what that was in case he came into combat. He turned into one of my favorite NPCs in a long time because making his stat block was creative and thought provoking. But you know, most of the time knowing class and level and knowing its a goblin cook is enough to run with it.
In general, 3.5e is a lot of work to DM.
I don't know. Compared to GURPS? Not so much. I don't notice that it is significantly harder to run than 1e, though it may be because I run it like 1e.
I think again that at least some of this is burden that DMs are imposing on themselves.
My current campaign has been going like nearly 3 years now. The PC's are 6th level. Seriously. It's really highly unlikely that the PC's will ever be more than 13th-14th level even if we go 3-4 more year. By that time, I'll be ready to retire the story line and move on to something else. My impression is that when people talk about how hard 3e is to DM, they are usually running games where the PCs are like 12th level or higher - often starting out with high level PCs or if not rapidly advancing the character level. Are high level games tough to run? Heck yes, but they always have been. D&D traditionally has a sweet spot from about 3rd to 10th level. If you stick with it, it's not that hard to run.
The other thing DMs do to make the game insanely hard to run is bring like 30-40 splatbooks into the rules, bloat the character building rules to an enormous degree, and let their players run absolutely wild with system mastery and optimization so that you end up with a bunch of alternate raced, templated characters with 3-5 different classes casting a bunch of broken spells from 4 different class source books. Is challenging system optimized characters hard? Heck yes, but it always has been.
And yet another thing DMs do to ruin their own fun is go with fungible magic items, letting players get whatever the heck is on their wish list whether its the Christmas tree or whatever zany brokenness like 'Belts of Battle' some sloppy editor lets get past him, thereby bumping up their effective CR by 1-3 points and getting in an arms race. You don't want to be in an arms race with your players. You don't want to let the PC's treat magic items like they are part of their elective character build, because it's going to be a nightmare trying to challenge that with anything but Monte Haul decked out NPCs - and then the situations is quickly going to become asymptotic every time they kill an NPC and take their stuff. Wizards in particular get nightmarish if in edition to being able to select all their own spells (note, mine can't, they have to find them) they can load themselves up at will with the sort of protective gear that negates the problems of not being able to wear armor or use good weapons.
In short, yes, I see why you'd say that 3e can be tough to run, but I'd say 50% of the effort is elective to running high level games with lots of splatbooks and fungible magic items sold at magic-mart.
Again, I hear people screaming about "But those are the rules! You have to do that! 3e was built on those assumptions, otherwise it doesn't work!" And I'm like, "Wait a minute, aren't you the one claiming that the way you are doing it is broken?"