Actually, as far as I'm aware, only some GOG installers prevent you from using special characters, and those are installers for games where special characters in the path would cause issues, like Dungeon Keeper 2 for example. After all, these are old games, and old software often has issues with special characters in install paths.
The first time I encountered this issue was with the installer for Treasure Adventure Game, released in 2011. Meanwhile, the much older Imperialism does not have this restriction (both with 2.0 GOG installers). It seems entirely arbitrary whether installers have this restriction or not.
This just seems like a weird, stupid bug to me, more than an intentional restriction. At least I hope so, because if it was done intentionally it's even more idiotic. (Perhaps the programmer forgot that parentheses are allowed in Windows pathnames?)
But my main point was that I really wish they would just use a standard installer. This is by far my biggest complaint about GOG.
Some agreement on where to store save games would be appreciated as well.
Wouldn't Saved Games be a logical place?