for me the best argument against an ISD is still that it wasn't the first or one of the first big ships.
The best pro argument for it is that it is the most iconic huge ship Star Wars offers, and that's a point I think everyone agrees on.
So if I would be FFG and would plan to bring epic play with huge ships out on the market I would have started with an ISD, at least if I would think it would work and would ever plan to produce it. It would be the ideal ship to entice people to get into the epic play part. The fact that they didn't do that and picked 2 ships with some name, but far less renown (and from one faction) tells me that they won't don't do it at all. Also, the 2 rebel ships instead of a the more probable 1 rebel on 1 Imp ship combo might indicate that the respective Imp ships won't have the iconic value and they wanted to get started with known stuff (same strategy as with the small ships)
so independent of size and scale discussions, I would bet the huge Imp ships will far less known if not even new designs, but that's just me.