Thread pretty well tells the story: I have a sheet (a few years old) shows about 54 or 55 brand names, from 8 manufacture groups.
Generally, almost everyone uses Copeland Scroll compressors, however I see some private label compressors made by LG (to older copeland specs)... so far they are holding up well. Some say the Trane Scrolls are Copelands painted red.
The draft inducer motors are mostly from 2 or 3 companies, the blower and fan motors are all from Mexico or China. The capacitors used to be all Mexico, now some are China and a few high priced brands (none on new equipment I know of) are made in the USA (Amrad). Circuit boards are all from China, as are most contactors, sequencers, relays, pressure switches, etc. A few parts come from Brazil, where most automobile electric and electronic parts come from (nephew in law sells for that co).
Coil making is a manufacturing process... each co does it their way. Key is thickness of CO tubing... and QC during manufacturing. Some co's have gone from 3/8 co tubing to 5/16 or even 1/4 tubing... good and bad issues here. A few (Goodman/Amana and Trane/Am-Std) have gone to all aluminum coils... jury is still out on this. Nordyne group is trying 'micro-channel' coils... similar to automobile coils... LOTS of issues with this design... and lots of failures so far... jury is still VERY out on this.
Trane tried plastic blower cages in their furnaces, and some co's have tried plastic and paper thin metal blower wheels... lots of disintegration (flying apart) issues in the past.
Heat Exchangers (HXE) are either tubular or clamshell... good and bad points for both, depending on manufacturing processes. Metal HXE is made of matters... better units made with better metals... most co's higher end products have better metal HXE's.
Now what is left: How much better is one unit's sheetmetal than the other's? Well the joke in the industry is Goodman even uses larger SM screws... yet they still strip out. Goodman actually puts a part in their ne AH cabinet to strengthen the screw attachment point.
Having said this; I have visited 4 different manufacturing plants in the USA; and I can say AlliedAir has better QC than the others.
Personally, I have sold Trane, been a dealer for Goodman and Ruud, and currently am selling AirEase (part of AlliedAir; includes AirEase, Armstrong Air, Concord and Ducane, along with MagicPak; a thru the wall apartment package unit).
All AlliedAir furnaces come from the Lennox Marshalltown Iowa furnace plant except the oil furnaces. The coils come from ADP in Miss... Lennox owns ADP. The AH's come from ADP also... albeit sometime (nobody including AlliedAir corporate will say when), the plant in SC will start making AH's. The AirEase and Armstrong Air AC and HP and small package units come from Orangeberg SC as well as the MagicPak units... the Ducane and Concord AC and HP units come from Mexico.
I can also say the AlliedAir products are a good value. If one just HAS to have the latest and most exotic bells and whistles... well AlliedAir does not offer them. AlliedAir product ARE dependable when installed properly... Which we all know the installation is more important than the product.
The other thing I would add: The supplier you deal with is as important (or more so) than the equipment line. A good supply house can help your business, a bad supply house can break your business.