Here are some frame grabs from my head-cam:
There had not been much rain for a couple of weeks but the mud had not dried out enough to ride over the top using fat low-pressure tyres. And it was not watery enough to easily cut through to find grip below. Instead it just created a lot of gloopy rolling resistance making it far easier to ride down the 4x4 ruts whenever possible.
Another problem is the dead leaves that stick to the muddy tyres as you ride. These can build up and clog where the wheels pass next to the forks, frame, mudguards etc. Tyres can also grow in width until the leaves and mud scrape off on the chain and then travel around to clog derailleurs and gears. A good mud bike is designed so as to avoid these problems and so make mud riding fun, not frustrating or even impossible.
When temperatures dive below zero the mud can actually freeze to the tyres and frame. Even a bike with big clearances will eventually be immobilized by the sheer weight of the attached frozen mud. You will then need the equivalent of hammer to remove it.