I notice EGW and some others have dovetail red dot mounts.

I don’t know why but it seems these would be less secure than a flat machined surface. I know sights generally don’t move but then again they don’t have a comparatively heavy cantilevered sight hanging onto it either.

Am I wrong? Are these a cheap alternative to machining?

I saw Bruce Gray’s comment about red dot sights holding up to 10mm so I’m starting to look around at possibly putting one on a P220.

These also generally position the optic too high on the slide for use with backup iron sights. And they're generally not compatible with most holsters, even ones with "optic cuts".

Fine on a range gun, but not ideal for a serious use handgun.

I have a Venom Vortex mounted in the rear sight dovetail of a Colt commander 9mm. It hasn't moved in 2 years. I do not carry it, strictly a range gun. It is extremely accurate.

I've watched quite a few beginners run a season of competition on a dovetail mount and no issues. The forces are perpendicular and I wouldn't worry about that part of it. But RogueJSK has the real issue. With a dovetail mount the optic is up high and you can't have any chance of backup irons and the optic picture basically sucks versus machining the slide. Both are fatal for serious use in either real life or competition. FWIW>

The weight absolutely isn't an issue; the red dot and mount combined weigh only about 1.5 oz in each of my set-ups below.

It may depend on what you are using it for. Dovetail mounts do sit higher than machined, but some of them aren't that much higher. I use them on range guns and I'm not at all dissatisfied with the height of the dot. It is definitely much cheaper than machining and if you think you may ever sell the gun, being able to return it to its original condition is nice.

Here is my P210 and CZ Shadow 2, the P210 mount is a little high but the CZ mount is quite low.