I mentioned this a while ago, but one of the things that really underscored how goofy TM-related balance can be was Toxic's distribution in earlier gens.
In a lot of the early games, a ton of Pokemon could be taught Toxic via TM regardless of how much thematic sense it did or did not make, but the Toxic TM was usually a true one-off: one found in the quest, with no way to buy or farm more. This makes a decent bit of game balance sense specifically for the quest: the player gets to choose only one party member to teach a pretty unique move to (lots of other moves can inflict other status conditions, but only Toxic inflicts its unique brand of poison), but he or she could choose pretty much any party member to be the recipient. There's a trade-off.
Later on, once TMs were infinite-use, something like Toxic suddenly looks completely out of place. Now that the supply was no longer limited, it really draws attention to the fact that pretty much everything could learn it and how little sense that makes.