If it is anything like the New Order's, Marriage Encounter, don't go.
Puhlease.
We are talking about the SSPX here, so your advice is absurd.
It is NOTHING like anything in the Nervous Disordo.
I've been to the Matrimony Retreat, way back in the day, and it was awesome!
In fact, Fr. Pfeiffer and Fr. Leith were our retreat masters, and it was an excellent experience.
I even still have the notes I took.