I'm sure at one point or another we've encountered the unfortunate circumstance of either a poorly made map or unfortunate terraforming making a mex spot unusable for the rest of the game.
Mexes and hydros are already limited in placement to very specific areas of the map, there's no need to check if the terrain would allow it to be built. I'm pretty sure this is possible since air experimentals ignore terrain when checking for construction. I've also seen a mod that does this, and I'll update this post if I figure out what mod it was and/or who wrote it.
EDIT: It was @Balthazar's mod: https://github.com/The-Balthazar/BrewLAN/tree/master/mods/BrewLAN_Gameplay/WonkyResources