The 870 has too many pieces??
Maybe the express is different but on my wingmaster counting the 2 pins and nut on the end of the tube I am coming up with 9 pieces in my head and the only small pieces are the pins.
Barrel, forearm, firing pin housing, firing pin housing seat, trigger mech, receiever, 2 pins and nut.
I have only needed to clean my 870 a couple times while hunting and both times were where I tripped in a swamp and me and my 870 both ended up in the mud and water. I just rinsed everything off to get as much mud out as possible sprayed some oil on the moving parts and went back to killing ducks.
Other than instances like that I can’t imagine why you would ever need to disassemble an 870 in the field, they are bullet proof shotguns.