To answer this, for me, is to disregard Ifukube's music in its entirety, as it is all nothing short of brilliant. Yes, there are instances of cues somewhat mis-used at times to some. And yes, Ifukube, especially in later years, re-used his themes a great deal - even ascribing one monster's showa theme to a different monster in heisei films. There are even non-monster themes from the past in later kaiju films (Samurai Saga's mournful themes can be heard in G vs. MechaG heisei.) But all cues are distinctly Ifukube, and grand.
That having been said, I'm a huge, and some might say blasphemous, fan of the G vs. Biollante score. I had seen a lovely anime called Sea Prince and the Fire Child in the early 80's (recently released by Discotek) and its score somehow stayed in the back of my mind. When I first saw GvB it bugged the hell out of me - where had I heard this composer before? Bear in mind, this was on a bootleg copy from the just-released Japan VHS, and the internet was still a twinkle in someone's eye. I did make the connection somehow, upon repeated viewings. Koichi Sugiyama scored both, as well as a lost-in-the-US anime called The Ideon - why did any of us get rid of our old bootleg anime? Although his interpretation of a kaiju score is thought of by many as inferior, I disagree (with the possible exception of the disco-style action cue at the beginning.) There is an elegance and subtlety to his score missing in other non-Ifukube efforts, such as Masaru Satoh's scores and Godzilla '84's - although I do quite like parts of that one as well.
Also, having thoroughly enjoyed his work on the heisei Gamera trilogy for Shusuke Kaneko, another favorite of mine is Ko Otani's GMK score. It's a Spielberg/Williams thing - Otani's music just suits Kaneko's style. Otani's music for Kaneko's psi-fi film Crossfire (Pyrokinesis in the US) is also far above genre standards.
I guess the greatest compliment to be paid to these scores is to say that the music stands up well to repeated listening, and is apt to surprise in some little way every time.
That's my two cents.