12cm is pretty uncommon, I think, even back in the day when longer lengthenings were more common.
With regard to Betz etc 'letting' people lengthen those amounts, I think there are a couple of factors.
1) 5-8cm often isn't a number a lot of people who are willing to do this surgery will settle for, but quite often 10cm is. So rather than stopping at some arbitrary number like 7 or 8 and then having to do another (super invasive) surgery to be satisfied, it's better to just go for those extra couple of cm and just recover and move on with your life. You can debate the merits of this but trust me, your world looks different when you've spent a couple of months in pain and discomfort lengthening your legs.
2) The Betz nail goes to 12cm and is manually clicked rather than using the machine to lengthen the nail, so he can't actually stop you doing more if you want to anyway. If I recall Apo's diary correctly, he wasn't too thrilled about him doing 11cm on tibs.
I'm also not sure where this whole 8cm max has come from, maybe because precise has (or had) an 8cm max, but I don't know if there is a hard and fast rule that at 8cm you'll be ok but at 8.1 you're crippled for life. In the cohort I went through with, there were a couple of guys that went to 10cm and they're fine now and that was 7 years ago. I remember Tall saying he did that second round of lengthening to stop arthritis but I think it was just him justifying another round of lengthening. More likely his leg proportions looked terrible and he wanted to even them out a bit, which is also a pretty valid reason if you ask me.
I personally don't think it's a good idea to do 12cm lengthening on femur even if you're one of those people that can tolerate it, unless you're going to do big numbers on tibs as well, and I wouldn't recommend that as most people just take too long to heal and it's no fun not being able to walk properly for months or years. Plus then you're arms will probably look weird unless you're starting from a freakishly long wingspan. All just my opinion though, but I'm one of those people that thinks proportions at least matter a little. I did 7.5cm on femurs and just that amount annoys me proportion wise. The only reason I'd do tibs is to sort that out and not because I want more height.