What compromises do you mean??
It all centers around the poor air handling quality of the systems. There is no way to mix outside air with the "airstream" of ANY of the aftermarket A/C systems. Essentially the aftermarket is running in Recirc / Max mode all the time. They keep recirculating the cabin air and the only way to introduce outisde air is to open the window. This doesn't sound bad at first BUT after awhile it becomes a big PIA. I did not think this would bother me at one point in time but after having one for a while it did.
Another item. The defrost is so-so. Air handling thru those flex hoses I suspect..or is it the blower motor capability? I don't know which it is. The evap core and heater core are small in the aftermarket systems. I have one here I am getting ready to junk and I just might cut it open to show the size of the evap core and the heater core and evap core from the aftermarket compared to an OEM Chevelle evap core.
Another item: my buddy has a VERY VERY nice 34 Ford Street Rod. I know I know we are talking about Chevelles but this is an example of the air handling issues with the aftermarket systems. When you first fire up the car if fogs the window until the system stabilizes the inside temp and drys off the inside of the windshield. Again..a show stopper? Might not be but is it a PIA? Oh yeah because you are NOT going anywhere until that window clears. The 34 has been charged and recharged trying to solve this problem and it just won't go away. Nuff said there.
Yes you can shut of the hot water to the heater core to assist the A/C output and help with the fog situation BUT the heater core and the evap core are side by side inside that tiny little box under the dash. Heat from the core WILL effect cooling even when the water shut off is closed as witnessed by what happens when you first fire the car up.
Cooling. There just is not the cooling capacity that the OEM systems have. Will it be cool..yes. Will it be COLD? No. The aftermarket outlet temp might be 40 degrees but the evap core is a heat exchanger. Here comes that little evap core in that little box again. If that evap core cannot take on the quantity of heat inside the car, and get ahead of the heat, you are always trying to cool the car. The 34 is TINY and I mean shoulder to shoulder tiny for two guys. Much smaller than a Chevelle. The car is insulated up the wazoo / tinted glass / steel body...all the Street Rod tricks and it just NEVER gets cold it gets cool. Cool ...yes, cold no!. That just won't cut it for me. Some folks have different toleration of the temp level. People love the aftermarket stuff and I am sure there are proponents here in this thread already. To each his own. For me it is about what is acceptable to one guy versus another.
I have had two aftermarket systems and I just went back to an OEM system recalibrated for R134. It is just a general feeling: there are no glaring flaws with aftermarket. When ya stack up the issues I have outlined and other annoying things like getting a part if ya need one it just doesn't work for me. Some aftermarket systems are sealed...you can't crack that little box open and get inside. Again is it a showstopper? No it isn't until the heater core croaks or the evap core leaks or a servo chokes. Then what? Then send it back to the manufacturer because they are the ONLY people who can service it. I don't like that fact one little bit.
IMHO anyone with an OEM A/C car who goes with aftermarket has stepped backwards. I also don't subscribe to the clean firewall mantra. Sure the firewall has the evap core on it in the factory cars. I ride inside the car not inside the engine compartment so for me cool dry air where I ride is more important than a smooth firewall in the engine compartment.
In point of fact the car I just did was non Factory Air and I converted it to factory air rather than load in aftermarket. Was it easy to do? No. Will people tell ya it cannot be done ..certainly, but I am here to say I did it and it works. I did it without welding on the car. Not going into that long story here. Folks won't believe it anyway. Suffice to say that converting to factory air, even considering all the work involved, was for me preferable to all the work loading in aftermarket air then having aftermarket air when I was done.