While I think your point about katana mags is totally valid, honestly I would probably only use a blaster like this with them. The reduction in deadspace is a huge advantage in my opinion, and buying katana mags isn't that much of an issue to be honest.
Also, while you may have had a good time assembling the caliburn, you can't deny that the rod system is a little janky and kinda hard for amateurs to assemble, while a receiver with slots pre-milled would essentially be idiot-proof.