Formula One ( F1 )

^ From that article:
"The cars might follow each other better from getting more downforce from the underfloor than the upper surfaces..."

Why has downforce generated by the undercarriage always been so controversial? Lotus had that skirt design that was banned in the 80's so is there some rule saying the undercarriage must be plain or uniform across all teams?

Yes there are rules about ride height, floor design and permitted wear of the standard plank. These are all to minimise ground effect. The big worry came from cars getting so close to the ground, bottoming out and then completely losing grip and skidding off. Similar to what happened to Senna (according to Williams).
They also try to limit/prevent ground effect as it is a hugely efficient way of producing drag free downforce which makes the cars very fast. And we can't have fast cars in F1 now can we?