Man it sure looks cool. I've seen one at a range before but I didn't get a chance to shoot it or anything. I talked to the guy shooting it and he said he liked it, not something to run around with, but as far as the scope rail being part of the chassis there didn't seem to be any problem. I'm sure it will be fine if you install it correctly. As far ergos go... I'm pretty sure it's going to feel a helluva lot like an AR-10 no?
My XLR uses AR furniture and it sure feels like an AR, I even accidentally reached for the AR safety selector with my thumb a couple times. The only weird part about the TAC 21 for me is that it won't let me place my thumb on top to pull the bolt up with my fingers (my preferred method of runnin' a bolt), with the Tac21 you pretty much have to grip it and rip it, just as one would with a Ruger RPR.