When it comes to blowback there is simply no question, bb currently wins every time over pellet. There is no need for the complexities and workarounds used in a pellet firing blowback where as the system can be simple and works well for bbs.
The PX4 is probably the best of a bad bunch, but I feel that we are missing out on what could be awesome pellet firing pistols. It is something I want but I think that until they employ proper, non rotary magazines containing the CO2 they are always going to be a gimmick.
There are engineering and design challenges, they are fairly simple to overcome but will cost slightly more to produce (until they become established). They may not even cost much more considering the lengths they go to with current feed systems.
Great Deals with : Dave Dee, Dozy, Beaky, Mick & Tich