Even if power weren't a concern, Opening Salvo wants to be a rare just to help sell the set (at least for Limited play)
The player controlling Solitary Confinement must discard a card every upkeep or else sacrifice the enchantment, and the controlling player has to skip their draw step
Regardless, nearly every 'attack' in the Pokemon TCG must be powered up by Energy Cards
If you control two Roaming Thrones with the same chosen creature type, triggered abilities of other creatures you control of the chosen type trigger three times
Because theres a smaller selection of them, they tend to be situationally good, or theyre really more viable in a different format
While there are a plethora of Summon cards to suit various types of decks, a deck dedicated to summoning and summoners is viable in its own right