This really isn't a good cheat in card
A summon is a saga creature, so it works exactly like a saga: Once you reach the final chapter, and its triggered ability resolves, you have to sacrifice the saga
Sure enough, the English version they are almost completely washed out: This may be information known by others, I couldnt find anything with a quick google search, but figured it would be useful either way
The broader target range and better late game scaling make Oust the better control card but both are good enough that you will play either in either if you just need more removal
Opus highlights abilities that trigger whenever you cast an instant or sorcery spell, and each opus ability has an initial effect as well as an additional/alternative effect if you spent five or more mana to cast the spell that triggered opus
Overall, the meta will likely shift significantly in 2025, and Wally seems poised to be part of the reason why