When used in a Dynafit, bellows boots sag while in alpine mode and this not only can cause premature release, but changes the binding DIN setting depending on how much the sagging changes the position of the boot heel on the heel pins. This is prevented somewhat by attaching a “puck” to the ski under the forward part of the boot. Problem is, the boot sole ends up resting on the puck, which can introduce possibly release compromising friction into the otherwise beautifully friction-free Dynafit system (normally, the boot is suspended between the toe and heel units.)

Personally, while I can see that bellows boots might be slightly more ergonomic for lower angled touring and for walking around the parking lot, I don’t obsess on them. I’ve used them extensively (I have F1 and F3) and find that they actually take energy when used with binding heel lifters, as they sag at the bellows each step. And when used with crampons, while doing front pointing they’re definitely not as efficient as a stiff soled boot.

Where they shine, again, is in lower angled touring. That’s why I keep some in my quiver, and is why they’re popular with the race crowd, because most rando races do not have much steep skinning. Instead, racers use glide optimized skins and sometimes almost look like nordic skiers doing classic stride.

If your goal is to telemark and AT ski in the same boot, perhaps they have their advantages there. I don’t telemark ski, so I’m not up to speed on that aspect. Though I do have to think that any core telemarker would want a dedicated pair of telemark boots that are set up specifically for telemarking…

Fact of the matter is that building a bellows plastic ski boot has always been a challenge. In terms of doing it for AT skiing, it’s pretty much a solution without a problem — and just adds cost and complexity to an otherwise elegant solution.