I haven’t really had any bad experiences with group buys. I’ve really only been in the hobby for a couple of years, but I’ve been involved in a number of keyset buys, and the only problem with those has been the occasional minor delay.
The Grid 600 buy has been the only keyboard GB I’ve been involved in. There was a bit of a delay with the white cases on that one, but the biggest disappointment was that the colors of the anodized aluminum modules ended up being completely different from the renders. So if you bought hoping to pair them with a particular keyset, you were probably disappointed (as I was).