Actually the icon size limitations can exceed 32x32 now, so there should be no limitations on that.
There really is no limitations in Byond, except how the network interacts (it's primarily a network game engine), you just need to read the documentation and learn how to program in it effectively/efficiently. It's comparable to learning C++/Java, but it has a lot pre made processes and whatnot to give you a bigger jump into making games.
Btw, what exactly do you mean by customization? It's very customizable, but like I said you need to understand the programming language.
In your case, what exactly is your limitation?