This will account for VGC, Doubles OU, and Singles OU.
Btw here is the site where I get my info from: https://pikalytics.com/pokedex/vgc2019ultra

When looking at the VGC and Doubles OU highest used mon, it's often overwhelmingly Incineroar (86.18% usage rate in VGC 19 that's fucking crazy) who isn't a mega, and often isn't using a z-move. Instead it's figy berry or assault vest.
The next highest ones are the legendaries for VGC, which are self-explanatory, nothing really special to note here.
Doubles OU and VGC have the Tapus which made up a good chunk of the top 15 most used. The Tapus themselves didn't even really use z-moves too often aside from maybe Koko in Doubles OU.

Now for Singles OU. There are no currently non-banned megas in top 10 usage whatsoever. The most used mega in OU doubles is Mega Mawile with a rather middling 11.37% usage rate. To boot, Z-moves weren't even often the most used items for a lot of the highest usage mons in Singles OU.

When looking at the usage stats for megas, and usage of z-moves in all of the metagame formats (VGC, Doubles OU, and Singles OU) quite a number of them don't even usually make even more than 25% usage rate at best. (Unless you're Mega Fug, who gets to hold an item, or Lunala with that z-move)

Yes it is true that megas, and z-moves can each only be used once on a team and that would contribute to a much lower usage rate as a whole. But remember to use either one of those, it has to be held like an item and that is a significant cost to using something that might be easily countered (megas) or be rendered completely useless (z-moves).

Dynamax however, an item can be held while dynamaxing which might create some really gamebreaking scenarios. That's an entirely different topic though.

tl;dr megas, and z-moves are far from broken generally speaking, and don't make up a massive chunk of usage in their respective metas.