Trans service is removing the pan and changing the filter. If that is done at the recommended service intervals a transmission fluid exchange really isn't necessary.
Transmission flush is a misnomer. It is actually a fluid exchange because the pressure the transmission produces in the cooler circuit is what actually makes the exchange happen. It is not like the exchange machine is under enormous amounts of pressure and blowing the fluid though the transmission like a pressure washer. If there are machines out there that do that sort of thing, you should steering clear of them.
Even though the pan is removed some of the fluid is still inside the torque converter. Fluid exchange swaps the fluid in the entire transmission including the torque converter. Fluid exchanging isn't bad if you still service the transmission at the recommended intervals. Even when an exchange is done, you should still change the filter.
Fluid exchanging goes bad when chemicals are used or someone thinks a "flush" is going solve a transmission problem. If the fluid is dark and burnt, the damage is already done and no amount of fluid exchanging is going to fix that.