I'm normally a C++ programmer, but I think I can answer some of your questions.
First of all, DARKBasic can render 3D graphics. If you have a link to thier wweb page, check out some of the demos posted on their site. For what I've seen, they are very impressive.
Read this carefully! DARKBasic advertises itself as a 3D GameMaker, so you're asking me what are the limitations on the software. Compare this to a programmer using C++.
If you didn't know anything about 3D Game making, you would have to purchase the c++ programming language, since 3D capability is not included in the language, you'll have to find or purchase a 3D SDK such as DirectX, OpenGL, or FastGraph, or develop one yourself. This takes a lot of time and money. Once again, if you didn't know anything about 3D, you'll have to learn. This is either in the form of books, the internet, or through friends. This also takes a lot of time. Books and the internet cost money as well, at least $40 to $50 for a book, and in some cases, $50 per month for the internet.
Now for DARKBasic, 3D is already included in the pakage, so that can save you some money, (though you'll still have to learn some 3D basics), and DARKBasic uses DirectX out of the box, so setup time to use that is nil. With other languages, you specifically have to train your language on how to use DierctX, and while that can be a piece of cake to a trained professional, it can be a nightmare to someone just learning the craft. Since DARKBasic is based on the Basic language, it is easier to learn than C++ as well.
I hope that this information helps you.
Good Day!