Good heavens, they are pretty simple to take down, just note how things are put in before removing them so you can re-assemble them with no problem. There are some little tricks it is nice to know, but anyone with some mechanical knowhow should have no problem and any gunsmith who can't dis- and re-assemble one without instructions needs to take up something like rose growing.
As for not being designed to be disassembled, or Remington warning not to take them apart, that is plain silly. Remington put out a detailed manual on how to do just that. (See the link in SDC's post.)
I recommend not disassembling the trigger group or removing the magazine outer tube unless necessary, but neither should be beyond the ability of a competent gunsmith.
Jim