I've noticed a peculiar pattern when using URC's preprogrammed codes vs. learning the codes from the original remote.
Both the MX-850 and MX-900 do it, so I have to assume the problem is in the URC database.
When using the preprogrammed codes for certain devices, like my Marantz DV9500, the remote would have very poor response to key presses, almost like a weak battery or poor IR transmission to the device.
This was very noticeable on the on/off buttons, and the joystick buttons.
When I went back in and learned the commands from the original remote for the on/off buttons, and the joystick buttons, the response is now razor sharp and instantaneous.
It's a night and day, no contest difference.
Is there something missing from the codes in the URC database for some devices, or are some devices just more finicky in receiving IR codes from the remote?

The data base has some major issues as is evident from trying to use the 810 preprogrammed 'wizard' codes as well. The best thing is to learn in your codes (test them thoroughly), and then set up profiles/dummy devices/templates with the codes for the equipment you use most frequently, and use the preprogrammed IR database purely as a backup rather then a primary source when customers have random old gear and no working control (ie old beta vcr, or dat or something).