Many TV mounts are made to mount on 16" center studs.
Just use appropriately larger mounting screws and you'll be fine.
I've done many Ergotron wallmount installations and if you use the proper screws that come with it, it's very solid.
As others have mentioned, if the mounting locations are not ideal, then a piece of plywood properly fastened to the studs will work just fine.
I've also used the toggle bolt anchors, they're pretty effective when you use a good number of them, properly spaced of course.
Whether using a backer board or just the TV mount, I do always prefer mounting to studs whenever possible. Metal or not, they provide the best possible support.