In fact, let's get to the heart of the matter.
The ONLY thing wrong with this whole gathering is that the SSPX, which formerly had the full package of truth, is now watering down their position and considering various compromisers and compromised groups to be equal to them. This is a step down for them, and a dereliction of their duty.
Because even if this group got together and prayed publicly together -- or even participated in liturgical worship together -- there wouldn't be a problem as (virtually) all of them are Catholics. But that misses the point that the SSPX used to know better -- they were the witnesses against Vatican II and the new religion. The various "approved by Rome" groups were all considered a bunch of sellouts. And working directly with the Conciliar Church? Unthinkable!
Not anymore. That's the problem.