Are the panels so much thinner?
1.5" thickness instead of 2.5" thickness of a 2x3 frame
Difficult but not impossible to customize if you are willing to spend + very handy + house wall or other support system to work with.
I consider myself very handy but I am never going to cut through fiberglass and deal with that dust or do the riveting. Any change such as a bump out, groove, corner, etc is going to cost serious money and the hassle of schlepping panels to and from the store/factory.
It's certainly possible to do a 3 wall succah and anchor to a house or such - but it's more complicated than doing the same thing with a wood succah.
The solid cross breeze windows are great if your willing to pay for em.
That's the name of the game when it comes to panel succahs. You spend thousands on the panels upfront and then later need to make changes or mods, which cost hundreds for any little thing. The cross breeze windows are excellent - make sure they're priced in to the original configuration.