I have the entire crew at 7 stars except fost and Fotp at the moment. I completely dropped fost he's terrible and even worse on defense where he wastes a third of his actions using a useless buff.
What I've observed is though I get knocked back more than when I use my standard meta group (qgj, lumi, yoda, rey, dooku). My assumption is Foo and Fotp just die before ever taking an action on defense leaving the rest of the group 5 v 3 with the tardish ai.
My issue with Foo is his second abilities turn gain is 80% pointless. Turn 1 use his 3, turn 2 most your teams bar are 80 to 95% complete anyway, turn 3 the battles over and your teams bars are still 80% full so the turn meter gain never really comes into play. But if you slow with phasma first (which is normally the better move because on phasma turn 1, most allies have near full meters!!!) on his turn 2 his basic does low ish damage.
So despite all the "synergy" they honestly don't work together well beyond 18% higher chance to extra attack. Which is enough to make them strong..... when rng feels like it.
Also on offense you end up coin flipping with Foo sometimes, if his turn meter activates you demolish, if not you often have to endure the storm before you can react