Actually SSPX priests are validly ordained. The Church is absolutely clear on that point. That’s why SSPX priests can easily be incardinated into other orders or diocese if they so choose. They do not have to be (re)ordained. I have known two such priests.
They were excommunicated for ordaining people without the permission of the pope.
Unlike the protestants, their ordinations were licit (ie, performed under the correct form), however they are not valid. They haven’t been valid since Pope John Paul didn’t recognise them. That’s the problem. Everyone who was ordained by SSPX is not yet a priest of the Catholic church, not until they are brought back into full communion, and they accept the authority of the Pope.
The biggest effect of lifting the excommunioation is to bring their flocks back into the fold.