Which compromises do you mean, please?
the z system is great in theory. The theory being that whatever gas you plug into it will provide you with primary, secondary, and inflator access from one bottle. Super slick for the ability to just plug in whatever bottle/s that you want to breathe from and off you go.
Where it breaks down is that it is positioned on your back, you have no way of isolating one side from the other in the event of a second stage or manifold block issue, you can't see the manifold block in order to figure out what's wrong, you have to use quite expensive quick connects on there, and there is water ingress every time you make a wet connect which is lovely when diving in salt water
All of that to avoid a problem of 50% of the time when on backgas. In a cave environment the risk can be mitigated with proper gas planning.
Start the dive on long hose, breathe it down a sixth. You want to start on the long hose since the highest risk of failure is going to be in the beginning of the dive.
When at a sixth, switch to the short hose and breathe that down to a third.
Switch back to the long hose and you are on the long hose for the last push of the penetration. When you hit thirds, you turn the dive.
From this point, depending on the cave, the flow, etc. you can actually just stay on the long hose for the rest of your dive. You won't have balanced tanks at the end, but you are only on the short hose for 25% of that dive at that point if you're that concerned. Carry a transfill whip and you can balance them out before filling.
So, what does the manifold do?
Increases o-ring count and complexity which DIR advocates argue against for swivels/elbows on second stages. Let's assume using the UTD regulators, so you have 7 ports in total with 2 hoses. 14 total o-rings there. Can't get away from that since that is what you'd have in a normal sidemount system, but they do add an OPV to each first stage since there is no downstream valve to release pressure in the event of IP creep.
You then at 8 ports from the manifold. They balk at 2-3 for a ball swivel, but then put 8 on a manifold?
Add the isolation knob which has a couple as well.
You have a minimum of 4 extra hoses for the quick connect lines.
Depending on how you set it up, you also have to carry a spare second stage that has the quick connect on it, or use a quick connect on one of the second stages on the manifold so you can breathe direct from the bottles in the event of a manifold failure.
They choose to use QC6 connections which are only available in NPT fittings, so you have that extra potential leak point in the adapters to go from reg hose to NPT
So, add in $350 for the manifold, $80 in extra hoses, $400 in QC6's *for 2 bottles, add $100/bottle for the male side only*. You can lower the quick connect price considerably by going to the Omni-Swivels where the connectors with check valves are $54 each, so you'd be at $220 for the standard two bottles and manifold, and $54/bottle after that. Total cost is $650 with Omni's or $830 with QC6's, not including another either one side QC plus second stage or another two side QC for the primary regulator for bailout.
So, minimum of $700 OVER the cost of a standard sidemount system, and almost $1k over the cost of a standard sidemount system to go the proper UTD route, with all of the added failure points and complexity, and all it does is force the always primary donate peg into the truly independent regulator system hole.
