If you decide to weld your 80/20 frame make sure you clean all the joints. If you don't clean it the welds will be weak.
Some years ago our team did exactly that. Amazingly the frame survived the season but we were able to disassemble it with hammers

We literally "pulled" it apart, it was that weak.
80/20 is also difficult to cut. I would not recommend using it unless you have access to a mill. Cutting 80/20 on a band-saw or with hack saws will leave rounded ends, making butt-joints impossible. Without secure joints your whole frame will warp.
I think 80/20 is a great and useful material, it allows for easy adjustability and prototyping, but I would not recommend building your entire robot out of it.