no. not if done right.
we just brought an old buried pool back to life and had to patch some major holes in bottom and added some benches and a spa.
the plaster is what seals it.
I would ask to see their plan first though.
As far a durability against salt, it is down to what flagstone it is. Some will hold up, but a lot of it won't. Balance really isn't an issue because the drying salt on the deck is what does the damage. When it re-crystalizes afer drying, any that soaks into the stone makes it flake apart. The less impervious the stone, the better it does in this regard.
My suggestion on the "Plaster" is getting the best Quartz finish that wet-edge makes. I too prefer smoother over the pebble finish, and I agree, Quartz will last plenty long. We chose a top shelf Quartz and we are very happy with it. Wet-Edge is a great product.
Of the three, I would choose Wet Edge. It's arguably the best product on the market, certainly as good it gets. It was started by guys who left PebbleTec and like their products, only approved applicators can apply Wet Edge.
If I had the choice of those, it would be my first pick.
We have pretty much decided on quartzscapes due to price. I have seen a couple of pools that he has done and they have turned out nice. He said that instead of a acid wash he fills the pool and drops the ph and brushes it a couple of times a day to remove the plaster over the quartz. Is this an acceptable method?
Yes just a different kind of acid wash. That's how ours was done. Not sure if it's less likely to get streaking that way but it's seems possible because of the distributed nature of the action versus concentrated. Both methods seem quite popular here.