RPM isn't bad, quite on the contrary it's good. However, and I'm well aware it's subjective, I don't feel it's a magic system fitting GURPS. Its mechanics are flexible and that's what allows it to be flavorful but it's also very vague, with not cold hard numbers to guide decisions as the rest of GURPS.
As I said, I don't think it's a flaw exactly. GURPS itself has its own downsides (eg Damage Resistance cost on high tech worlds, Mind Control's flat cost on high point total supers games) but other magic systems usually has similar flaws which makes it easy to see them all as one big family, but RPM is such an oddity that it feels like a stranger, while Incantation Magic is sort of a bastard between RPM and GURPS.