i ride in grass, mud, snow, pretty much ANYTHING except for ice. i also make sure all my horses are prepared with proper conditioning as well as either barefoot or the right shoes (depending on each horse/conditions/footing and their individual needs).
i've done some pretty gnarly hunter paces through mud, marsh, fields and wet grass with some rather large obstacles barefoot as well as with shoes and cleats. if the horse is conditioned properly and is used to being on various footing and you are a balanced rider there really shouldn't be an issue. if the horse is never turned out in the rain or mud however (as an example) that horse will need to be introduced to the various footing types and conditions gradually before you can expect the horse to be competent enough to balance, carry himself and a rider, and jump.
i spend the winter doing a LOT of trot work with very little jumping. this allows my horse to build tons of muscle, topline, and suppleness so that come spring i don't have to worry about building his conditioning, i can just focus on training for events.
hope that helps!