Farner had strong pipes (bit on the dramatic-intense side sometimes, Brewer's singing was more laid back) and an earthy, more rhythmically-oriented approach to guitar rather than just providing a flurry of notes at high speed à la Alvin Lee, but saying he wasn't a good guitarist is like saying Keith Richards or John Fogerty can't play. He also left room for his band-buddies Mel and Don. If you believe that Farner is a grooveless, unmusical guitarist, you're deaf. He was even sparse, but since when is that a bad thing? Free and Bad Company were always lauded for it by critics.
I believe critics had foremost issues with Farner's bare-chested and chest-beating macho approach, but hell, GFR came from a blue-collar, industry town after all. If you were from Flint, you weren't going to read poems on stage.
Speaking of "GFR without Farner", I thought the post-GFR-split spin-off without him monikered Flint didn't sound bad at all - it sure lacked Farner's overt drama, but the typical GFR groove remained very much intact (no wonder with the rest of GFR in Flint), their music had even more an RnB influence.