Perhaps not much difference at least in the UK, however motorways tend to have less junctions, so you need to concentrate to not get a tunnel vision after a while. Many dual carriageways are often two lanes, some motorways have three lanes, or the M25 even more.
And anything to drill in some lane discipline is good in my books.
Byways …
Thems are bridle paths that are a little gravel strewn if we’re lucky.
Good point by Art.
It’s not just Motorways that learner drivers don’t experience under instruction.
We might take them in our stride now but neither my wife or daughter have had much experience of iffy road “surfaces”. They have now.
Certain of the byways around here I would think twice about doing on the SM, let alone something like a Yaris! 
After passing my bike test, my instructor simply covered the L plate and had no hesitation telling me to follow him back to base via the M1. No advance warning just follow me and make sure you do your lifesavers - I was so glad that I had got that scary moment over before I even had time to be worried about it.
Snap!
SneakyMcC You’ve obviously missed the high life on the M1 and M25 at those times in the day when the Eastern European truck drivers are running in packs and not bothering with their mirrors. (Not that the standard LHD mirrors show much of RHD roads.)
There are lots of things you learn. With luck you do that by luck and some form of osmosis so you probably don’t notice. It’s called “experience”.
Newby drivers don’t have that.