On the week I kept those stats, it had a 98% win rate on 3-trophy pvp. Defences at that time were a bit more varied I guess. Follow that link for more.
I did enjoy taking that loop team against the divine meta when Ubastet first appeared, and don’t remember losing much if at all with it - Mercy tends to mean that if the board favours the enemy, it also favours me and I get to cast first.
Nowadays I am also trying to use the hero as much as I can, and currently levelling my Assassin class. That’s led me to some fun trying new builds and some quite effective teams. But nothing much use against the main broken Priest+Ubastet build, so I typically skip those and do the 1 or 2 trophy match.
Completely disagree on that part. Adding Mercy to the mix both for empowered and for her cleanse makes it far far better, faster and reliable than other loops, in my experience. The build you mention has no acceleration so relies on filling Alchemist naturally. Ymmv I guess.