Speaking with W126 knowledge, only. It was not hard to clean my evaporator but it did take some time and tedious work. It also helps if you have small hands.

I tried the coil cleaner method but I was not impressed with the results. It did not remove much, if any of the debris on the evap fins. I then started a manual approach with a brass brush and my arm inside the AC box. I got good results on the surface but I could tell that more debris was inside the fins. At that point, I decided to use a garden sprayer to soak the evap core in drain cleaner. I would dilute it 50/50 next time because it started working too well. There was steam/smoke pouring from the vents as the drain cleaner digested the years of organic matter lodged in the evap core. Since it started getting hot, I decided to apply a generous amount of water to dilute and wash away the drain cleaner. In the end, the evap core was shiny and clean. The AC performance increased about 15% in cooling efficiency.

I also vacuumed the evap core box out using a heater hose attached to my vacuum cleaner. There was a lot of debris in there.

The lesson is that if you use drain cleaner, dilute it and don't let it sit for too long but it works a lot better than evap cleaner.

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1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge