I don't think mud itself is too much of a problem to be honest, you see farm used vehicles literally caked in the stuff but once washed off they're still in solid order.
Its road dirt mixed with salt that's the real killer, get that salt trapped inside and its going to to do its thing without let up until someone eventually hoses the stuff out, and we all know that very few owners who can afford these new have the slightest interest in doing that, which why we are where we are.
During my previous work i saw the results of salt left on and what it could do to the most modern of vehicles.
We'd collect ex fleet/rental cars by the truckload and if they'd covered the required mileage too quickly (was 12k miles at one time for renters) they would be stood in compounds for months on end to balance supply/demand and keep values healthy.
Things were ok if a car was defleeted in October say and stood over the winter, fine underneath, spring summer rains had washed much of the salt off.
But if a car was defleeted in March and stood 6 months, that road salt coating underneath would have been burning into everything corrodable, load it up again in October to go to auction and the difference underneath on a car still not 12 months old was staggering, exhausts rotten and an all over white salt covering that the summer heat had baked on, especially on the brakes, i bet if you followed the history of such cars and knew the dates of standing involved you'd be able to predict which would last and which would have constant rust and brake issues.