This right here is the very reason why these different type tera Pokémon are not very good. They have good abilities and attacks but are trying to help one Deck while belonging in another if that makes sense. Being one type but using energy from a different type makes them less usable in my mind. How good would Mewtwo be if it was just Psychic type? Charizard ex could use Magma Basin if it was Fire type(I know that's not that big of a deal just an example). I wish the tera Pokémon that change type would use the energy of the new type as well.
Charizard definitely got the better end of things given dark type is great offensively and grass type is a great weakness to have rn on a Pokémon that big.
Mewtwo bring electric is not helpful, if not straight up hurtful rn.
Froslass being grass would be of benefit if it did at least 170 damage. That’s more the card being bad on damage than anything.
Eiscue bring fire would have been useful coverage eventually… if Chien Pao’s whole gameplan wasn’t already around trying to one-shot indiscriminately. Probably better to just build to a consistent deck unless eiscue’s attack ever became needed for disrupting something important. Maybe Palkia eventually needs it but it definitely doesn’t rn.
Vespiquen is honestly worse being psychic right now because you just exchanged one-shotting Charizard for… one-shotting SS Urshi VMax.
Garchomp being water is a worse type offensively, but a better type for Irida.
Steel Skeledirge is slightly better, but not to much of importance rn.