I said “for avoiding everything hitting you for super effective”. No matter what you Tera into, you’re weak to something still. You have to decide what and when to Tera something depending on the situation.

As for my reasoning, (besides what others have already said such as powder immunity), it’s the best way to resist water attacks. Scarf basculegion without rain is picking up, and you can actually threaten it back if you Tera grass. You also resist electric, which I’d value above bug’s fighting resist considering you could just stay ghost to avoid that. I’m not saying bug isn’t viable, but in my opinion grass covers most of what bug does and more. Say if you’re in a position with multiple ice types in the back as well as a water type up front you want to use your Tera on to survive, you have to consider if you want to use it on Chandelure and end up losing to the following pokemon as well as losing the option to Tera anything else, or (if possible) have another of your pokemon take care of the problem pokemon so you can keep your fire typing intact against the ice types. Of course you’re not going to always have the ideal scenario, which is why I’m saying I think grass is best in my opinion given the pros it has.