My only concern about the QT, assuming the guidelines are followed, is with the fit of the piston. I'm a big fan of pistons (based on early experience with PML pistons), but I like my pistons to have a close tolerance and not "rattle". If you have to sand the piston so that it fits in cool weather, than it might be loose normally. I've only had one QT rocket and it was long ago, and I don't recall if that particular rocket (a Phobos) even had pistons. But I hear about the temperature concern a lot.
One thing that I've learned in rocketry is that most people will tend to build and fly larger rockets over time as their skills develop. That may or may not be the case with you, but if that is taken as the general trend, then I try to advise people to avoid rocketry dead ends. These are things that you might do for a particular rocket that will never scale up beyond a certain point. So, why develop skills with them if you are more than likely to outgrow them. I buy lots of stuff from PML. But, PML sells some dead ends IMO. CPR is one example (learning to design and build altimeter bays that will allow redundant deployment is a better use of time) and QT is another. No problem using them on a given rocket for whatever reason.
Jim