Depends what kind of stability you mean.
I haven't had one herm on me and I grow outdoors, but as far as genetic stability they are not very stable and there can be quite a difference in phenotypes.
In my opinion fastbuds is for autoflowering or fast flowering (fast flowering are 50% autoflower). For photoperiod I would go for SweetSeeds, Barneys Farm or Ethos.
Happy Growing Buddy! 💚
Thats awesome. I would like to try some. If they are anything like the blackberry auto I just grew should be amazing. They should be ballsy though, and call they photoperiod line slow buds =]