I've changed the fog on the main menu and I've done proj. mods and hud changes using RTH, I think I've also changed the boost speed on the ghost but whenever I poke meta on the warthog my game freezes when I go near it.
Ones not to edit are the warthogs vehi tag and bipd tags and anything to do with the actual map because that will likely make the game freeze.
I wouldn't use entity because the rth is bugged, I would use mini-rth which needs an already modded map then you can poke the meta of something on the map and it will change in your game, just mod a map using entity then save and use mini-rth.
Also never swap coll tags as this will freeze your game
Swapping/editing mach tags work on most maps I'm not sure for all of them but it shouldn't freeze your game