142x12:
on a Fs, I agree completely, I had been using bolt up hubs on my hustler, and when I put some qr wheels back in there, there was a lot of flex.
But on a Ht, it's not an issue, at least, I've never had a wheel slip in vertical dropouts.
142, the 2 souls ones would look clean, tho the brazing is a bitch.
price is about 80-90eu more for the frame.
I'd prefer the adjustable ones, it adds a new dimension to a frame, you can SS it or rohloff it, or change between 135 and 142. weight goes up a bit, price goes up by 60eu or by 90eu if you want them in stainless,
so it's not too bad.
for a bike that won't ever be used in those ways there's no point though, if someone wants a rohloff bike the cable routing is different anyway.
if someone wants a ss, they're not going to want the cable guides.
I'm not trying to hit a pricepoint, there's no point on a custom bike, you can add options and have a custom price, but it's about the best frame I can build, price is always a factor that you have to consider however.
price brings me onto internal cables, internal cable routing in steel is tricky, you have to braze in a brass tube, then file it smooth, it adds about 50e per cable, per tube, so consider 3 tubes in the tt (dt internal is a worse idea)

plus you might as well go all the way, internal dropper, internal in non drive seatstay, internal in chainstay. that's 300eu more. don't know what that is in canadian, but it's an ok wheelset if you build your own, or find a good deal.
The formula Rep was here yesterday, we were talking about this, he says racers prefer external, changing parts is much quicker.
Non racers prefer internal, for the reasons stated here; cleaner to the eye, easier to clean,etc.
Another negative,
It adds weight, I'm not a Weightweenie by any stretch, but you're looking at 250-300grams more depending on framesize.
back to lateral stiffness.
Lateral stiffness is a question of tubing choice on a ht, more so than a fs, where you need to keep various suspension bits aligned.

Ragley uses 16mm chainstays, I have another solution, I used it on the 29er, where I profile 19mm chainstays to 16mm, so that laterally they're 22mm. bit more give vertically, bit stiffer side to side. (static load tests give me (aprox) 12% difference either way, not much, but it all adds up)
changing wheelsets.
those syntace shims are the best I've found, much easier than the individual bolt shims.