ost large 220v AC units use "contactors" to do the power switching.
That's totally true, and I'm starting to wonder why. I used to have a bunch of contactors around the house. The water heater, well pump, pool control, both AC units, and I've been replacing them with SSRs as they start to have trouble. The well pump was a real problem since it was outside near the ground in an enclosure and ants would get caught between the contacts as they looked for food. Then other ants would smell the roasted one and come to investigate which roasted more ants .Bug poison would help, but the rain (rare) would wash it away and back came the ants.
After a short while, the acid from the roasted ants would corrode the contacts and I'd have to clean them to make the pump work. Then after about three times doing that, I'd have to replace the contactor, which is a high dollar item at the pool pump repair store. Now, with an SSR in place of the contactor, I haven't even seen a sign of a problem. Had to use a heat sink on it though since the combination of summer sun and current through the device got it too hot. I also had trouble with a contactor on the water heater. It was a 110V device and made noise. I found out that that is a really common problem with contactors, they eventually start to buzz. The SSR I put in there solved the problem nicely and didn't need a heat sink since it was indoors.
I only have a contactor on one of my AC units, so far. The contactor started to have trouble and needed to be swatted with a wrench every once in a while to knock crud out of it. I tried to keep the various animals that were pooping on the AC condensor and causing the problem away, but you can't watch it all the time and a cactus wren just never gives up.
I'm a big proponent of SSRs.