Which ones do some consider invalid and why? Mostly the Novus Ordo Rites or also the Traditional?
I’ve never actually heard this distinction in Orthodoxy, I’ve never heard of someone saying the novus ordo is invalid but the traditional rites are valid
the debate as far as I understand it is if you can actually have sacraments or sacramental grace outside the Church to begin with. in my experience, those who say no usually say no, just across-the-board.
and to preempt the possible question of whether this belief is donatism, I don’t think so because from what I understand donatism is denying that unworthy ministers IN the church have valid sacraments. The sacraments of schismatics (which from the EO perspective would be the papacy in general whether novus ordo, sede, or whatever in between grade), would be a separate question
tbh I don’t have a huge dog in the fight myself; im more than willing to admit I just don’t know; but I’ve seen people with very strong opinions on both sides