Ed, I've been running a concentric throwout system for the last 25 yrs. I adapted a Tilton racing unit to the ZF because no one sold such a thing back then. Two common fail-spots: when the mfg says 'establish 0.060" of clutch to throwout free play,' BELIEVE IT! The whole rig uses a single square quad-ring to seal. It seals on the OD of the cylinder bore AND simultaneously on the internal piston ID, and works only in-and-out.
What happens is, guys skimp or error on setting the required free play. When the engine & bellhousing are hot from 15+ minutes of driving, the free play closes up considerably. If it disappears altogether, the concentric piston bottoms and begins spinning with the input shaft. The quad ring will fail in 50 miles if it starts spinning. They are reportedly difficult to replace (I've not had to).
D Quella used to sell such a rig (not a Tilton) and reportedly had massive warranty problems, mostly because of inadequate free play during home installation, I think. Dunno where his system was sourced from.
The second problem is obvious: if ANYTHING goes wrong, the ZF must come out to fix it.
Advantages:
1)- the rig is 12 lbs lighter than stock- you don't need the stock slave or its bracket, nor the cross shaft in the bell housing, its bearings/ bushings, or the cast iron release fork. It all goes on a shelf.
2)- With a std bore clutch master, the concentric throwout will stroke in/out around 0.090"! So one has more than enough in hand for large amounts of free play, and it's self adjusting once set up properly. This is good for saving the usual ZF synchro wear due to stock clutch drag. Its also easier to push in & out: Judy (5'2" & 118 lbs) had no problem street driving it 500 miles to 'Vegas.
My rig adjusts by threading in & out on a special ZF nose piece w/ a stock nose seal. The bleed hose extends out one of the ZF cross-shaft holes while the pressure hose goes in the other. Quella's adjusts by a stack of shims on (I think) a stock ZF nose piece. Both systems work fine. Good luck-