I have done some a/c stuff this past tractor'ing season.
Three of the 5 that I dealt with were direct result of either shade-tree mechanic'ing or not know what was actually going on. Or both in one of those cases. All three of them were $2000+ to repair. One was charged with improper refrigerant, causing compressor failure and black death (replace every single component). Another was the fault of a leaking o-ring. Keep charging it every week but with no knowledge of how much refrigerant is in the system. It becomes overcharged, full of air and moisture, compressor fails, same thing. Third one had a pin hole in the condenser. They knew it had a leak but kept charging it every time it'd run "dry"...problem with that is, there is no way to know how much air and more importantly moisture was introduced into the system. Again, compressor failed and entire system replacement was required. Of all of the tractor jobs I did last year, that was just 5. Why not more? Some guys don't want to fork out much money to have their tractor worked on. After all, so-and-such down the road fixed the a/c in the car for $100 why's a tractor cost so much? Because we don't know what was fixed in the car....they are entirely different. Or perhaps the car mechanic is/was non-licensed MVAC repairman with none of the proper tooling needed. All owner knows is that car was broke and car now works. But for how long?
The systems are quite dangerous to work with and if you aren't positive that what you're doing is in fact safe/correct, you can kill yourself. I watched a guy working on a class A motorhome once....where the high & low side fittings were backwards (retrofitted improperly) and he half-heartedly decided that the refrigerant level was "low". Grabbed the small can of parts-store fix-it R134A, stuck it to the low side, turned the system on and the can exploded, badly injuring his hand and also permanently injuring his eyes with refrigerant. Refrigerant is nasty stuff. And there are two points made here: one was obvious, the second is that any work done improperly can endanger not only the worker, but also anyone down the line who might have to diagnose/work on the system. This is exactly why the EPA frowns on non-licensed people doing work on a/c systems.
Safety aside, there's also environmental concerns that are, in the eyes of the EPA, equally important. I hate the EPA but they do things for reasons and they ARE the government, so we have to go by what they want. Here's another great example. According to section 609 of the EPA's guidelines for refrigerant (specific to MVAC-which includes tractors), if I watched you knowingly discharge refrigerant to the atmosphere, I can turn you into the feds and I can be rewarded up to $10,000 for turning you in; and you can be fined up to $27,500 per day, per violation, then your name gets put on a list that everyone can see. That's how stiff the EPA is on this stuff; thus be real careful with what you're doing. If you ain't sure, hire it out to a licensed MVAC person.
One of the things that I personally do on tractors if they are in for servicing is to give the owner a choice between levels of servicing. If he wants to go full monty, in other words completely service the tractor, I'll go as far as pulling the evaporator out (or at least opening the box for access to it), and then cleaning it good with coil cleaner. Tractors get used in environments that are dirty (obviously). Dirt gets in/on the evap and heater cores, restricts airflow across them such that heat transfer doesn't take place as efficiently, thus either the heat or the a/c doesn't work as good as it should. This is common. That is one of the many things that can go wrong with a/c systems that is a simple fix, yet so many improperly assume that it's low on freon. If it's not leaking, it's not low unless someone's been monkeying with it previously-which is a whole other can of worms. Without knowing it's history, there is no telling what's been done...and many times whether it was done right or wrong or just halfway. That's another area that things can get real expensive real quick.