the only real limitation with learning the triggers to an MX850, is if you are dealing with RS232 devices and repeating commands.

there is a glitch in the msc400 that makes rs232 codes repeat unless you tap the button for a very, very short duration.  this usually isn't an issue if you are just repeating volume up and down commands, but if you need to navigate a menu using RS232, it can be a pain.

the work around for this is to create two smart macros.  one for the code to repeat, and another with the repeat option disabled.   then on your remote, you create a press and hold macro for the commands that you need to repeat.   a short press triggers the non-repeating smart macro, if held you trigger the repeating smart macro.  there is a tech bulletin (#10) over at the URC Control Room that explains this.

If I remember correctly, the MX850 can't do a press and hold.  So you wouldn't be able to trigger both commands with a single button.