I had to take ukemi on real ground a few times.
Not in 'street fights' but in accidents. Only two I recall o look like a roll. Once with a bicycle, once with roller blades getting on soft soil.
The first time I had to hold the bicycle in order to not get hit by it. In the other case I just held my one arm in a kind of bow, just to get formed like a ball and roll over my soft parka's backside through the little bushes.
On black ice accidents (again bicycle), where I could not move free and fell frontside down, I slapped with both forearms in order to protect my face.
I cannot say exactly, when to slap, but on rough ground I would generally avoid it, as the danger to get the hand or arm broken is too big.
Nor would I put my hands on the head as I trust in my ability to keep my chin close to the chest without hand support and for the hit itself the skull can take much more than my fingers.
Only If I have to go head foreward into bushy area, full of twigs, I guess it is very useful to protect your face - especially the eyes - with both arms, then yound hands ar on your head and the elbows in front of your eyes. But only if I cannot turn and go backwards into the mess.
I have some experiences from my time before I learnt ukemi and after that. Having the reflexes trained helps a lot. But I am far away from being an expert.
My first two days on a snowboard (2 years ago) were a ukemi desaster. Neither mae ukemi nor ushiro ukemi worked, not even the basics, which had nothing to do with my feet being fixed.
I should take more snowboard lessons to improve my ukemi, but I hate the headache.