DISCLAIMER: I don't like GURPS, but I still recognize it to be a well-crafted and powerful system. I just personally don't enjoy it, or systems like it.
GURPS is advertised as a universal system that can run any genre or play-style equally well. This is not true. What the advertisers mean is that it can work with any setting. However, the system is rooted to a very specific play-style: that of stimulationisim.
The combat system is very precise and blow-for-blow. Each turn lasts one second. There is a rule for everything. Everything is accounted for. I hate this.
(See also my rant about combat, hit points, and called shots
. It explains in much greater detail why I don't like the combat in GURPS.)
The GURPS system is so detailed, fleshed out, and comprehensive that there is little room to breath. There are rules for how many inches a character can jump, and how quickly they can think. This means that far too much time will be spent flipping through books, and there is little room for narrative freedom. Of course, one could simply ignore these rules, but at that point they would be better off playing something else.
Character creation in GURPS is a nightmare. It takes a very, very long time. Some derived statistics require a calculator to determine. The whole thing feels like it was designed by engineers, who were far more concerned with making an option for every possible scenario than making anything user-friendly.
One does not create a flexible system by making a rule or option for everything. They make a flexible system by including good advice and guidelines for making these rules and options on the fly. This is a far more powerful tool than artificially created comprehensiveness.