The Party

Something that the AD&D rules did well that GURPS never did at all was clearly define the roles for the party.

A dungeon crawl needed a cleric, fighter, magic-user and a thief.

Skipping one left a huge gap in capabilities and made everything far more difficult in a dungeon crawl.

If one recognizes the need for these roles to be filled, you can certainly make a GURPS character to fill them.

For some reason, getting the players to actually do it is like herding cats.  Being unconstrained by the rules to conform to the class norms tends to lead to characters that can do a little of each of the four required roles without being good enough at any of them to be of much use.

Jack of all trades master of none.

This problem comes up less in GURPS when the setting defines the roles; and aren't fantasy.

Space settings have no problem with one person being captain, navigator, security, gunner... etc.  Except the ship's engineer.  For some reason anyone who makes the engineer will play once and never bless the table again.

Military settings only have trouble with the officers and NCO's needing to spend points on rank and the associated skills.

Steampunk, Cliffhangers, Pirates... no problems with roles.

Fantasy where they need to emulate the D&D template... Chaos.

Even in throw-together multi-dimensional nexus worlds there's no problem with getting them to specialize.

It's a mystery.