The neck on my AB1 5 string has been completely stable. If it was going to have problems, I'd imagine it would have them here in the Texas Tropics.
I have heard of one, maybe two people having some neck issues, but they were well taken care of. Sheldon Dingwall takes very good care of his customers. Frankly, as solid as mine feels, I have to wonder if those folks went overboard with heavy gauge strings. Since these basses use extra long scale lengths, there's no reason to use a .110 E and a .135 B string - those low strings will be plenty tight with .100 and .125 gauges. On my other ERB's, I'll use heavier strings, but it's just not necessary on the Dingwall.
It's a very well built bass. Perfectly balanced, super comfortable neck, and of course one of the best B's out there. For my taste, the pickups have a bit too much "zing" to them, but mine has FD-1's; newer basses will probably have FD-3's, which I've read have a somewhat more vintage tone. That all depends on what you like, though. I tend to like darker sounding pickups. Some people prefer brighter pickups.