Mk1s with glass sunroofs are worth a lot less as the mod is not genuine and is irreversible. They do also tend to leak and rust if fitted badly. They look pants too and are best left to ford fiesta special editions!
Original sunroofs are still a weak spot on the rust front so the advantage of having one is often outweighed when it comes to value.
Nowt really to do with strength as few people track them anyway.