My hangup with Joel certainly isn't his playing, it's that I don't know him and I don't look to a Whitesnake at this stage of their career as a showcase for talent I don't already know, I want to see the older guys who either were part of their history (like Sykes) or alternately guys back from that era who are a good fit (like Aldrich).
I know that sounds a bit close-minded, but most times I'd rather see an aging old favorite band give it a best shot to perform their legacy with familiar faces rather than talented hired guns who may do everything right musically and perform well but I've got no attachment to.
Reb is a different subject to me, I love his playing but I don't like the Whitesnake support role, and I don't think the band is a great fit in general for him (even if he was in the "lead" role). Winger, Dokken, his very cool Fusion Demos album in the 90's...much better IMO, but I can understand the paycheck. Not like he's humiliating himself with some country crossover fad type thing either, it's still rock & roll.