Austria is pretty soggy too (and lots of snow). Having used an original Vélosolex for quite a few years in soggy Nantes: yes you do have drive slip.
But having said that when there is enough water for it to slip it only does so on bumpy sections of road and you don't want to be going fast in those conditions. The Vélosolex tyres were very also hard compound to stop being chewed up by the drive, compounds have greatly improved since then. The contact area was slick.
I had an accident (hit and run) with mine and stripped the motor down and modified it slightly (I was in my 20's
). While I was refixing the broken bolts holding the alternator (?) side back on I whipped off the cylinder and filed the ports and a bit off the top. Top speed well in excess of 45 kph maybe 55-60 kph, no speedos back in those days. And to go really fast you had to push the lever on the motor with your foot to force more traction on the tyre...