I think there are a number of posts here about the sound quality of performances in TV studios. I'm sure that plays a part, though I doubt 'Joe Public' takes that into consideration. In arenas, as part of a full show, I am sure the sound and overall vibe will be very different. I've not been looking on twitter for reactions, but it may be that the reactions are as much about the rest of the band as they are Neil being a sub for LB. The negativity surrounding Stevie seems pretty heavy. I definitely order my Mac vocalists LB>SN>CM (personal preference, not a judgement on their technical ability). I had a strong preference for LB vocals, but I always enjoyed Steve's vocals live (seen them 3 or 4 times).
Ultimately, some people don't like change, and despite how Mick would like to paint it (and it's his band, so he can do what he wants), I think people (normal people, not musos and super-fans) these days consider the 'true' FM lineup to include Nicks and Buckingham. It is a big change, and even if it turns out OK, it was unlikely the immediate reaction would be gushing.
I still find the sacking a strange development, when the band sans Stevie recorded and released an album together so recently. What isn't strange in the history of the Mac though?!