Older Resi equipment is probably the easiest stuff to work on. Alot of the newer units that contain more proprietary parts and require more extensive training is going to change the game. Companies who hack equipment in are going to be having alot more problems with these newer products. Just considering the enviroment you have to work on resi equipment and the lack of inspections i encounter is sometimes appaualing. I've refused to work on equipment numerous times because i knew i be married to it once i touched it. Most Commerical and industrial products are setup properly, wires and other things are labled. manuals are where they should be ect. If you know how to use your tools, brain and know how products works your on your way to the top as long as your attitude is positive.
I recently worked in a 1 year old gated community, 55+ in NJ. The furnaces/5 Ton Condenser was tripping on limit because at the lack of a bottom return plenum or at least having the return taking from the left and the right side of the furnce. Filter driers installed backwards, humidstats installed right next the the humidifers, A/C and furnace condensate shared the same drain so you could fill the heat exchanger with condensate if a backup occured, SP was over 1.0", no backup drains under each system which were located in a second floor maint room. I could keep on going...I told the homeowner to call the builder because the systems need to be removed and reinstalled properly. I showed him the manual for the furnace on my ipad because he though i was full of it. I made a list of the problems and he paid the bill. And the subcoolings was way off... (no caps on the schrader serivice ports) At least i hope thats the only problem but i highly doubt it.
Sorry for rambling.... but i see and have way more issues with Residential work then i ever had with commerical... Commerical can be more challanging but residential is more stressful. It does depend if you like to use your brain or not. Resi keeps you busy because the hacks make it rain.