Obviously, neither remote "knows" anything.
The Harmony gets programmed via a simple web interface, while the MX-500, for example, remotes get programmed through direct learning. If your display does not have direct input selection available on the remote, you may be in for some trouble when tryng to access direct inputs via a programmable learning universal remote (any of them). The Harmony has an advantage in that they have a well-maintained database of remote codes (some which don't even make it to the actual remote controls.)
If you want "simple" (and it sounds like you do) go with the Harmony. You may also look at Pronto-style remotes, which many rave about but I find less appealing due to a lack of tactile buttons, save for volume and some minor controls.
We have touched base with Universal Remote in the past (makers of the MX series) and have told them they are losing out on not having a more updated database of remote codes, but I think they are mostly focusing on the custom install market - to which they absolutely excel, though at the expense of other potential customer demographics.