Per the documentation for the Grid's Search Panel:
"When the server operations are enabled, you can search only by using string fields."
This is an oddly-specific limitation to have that causes an awkward user experience. Grids in some areas with limited data might use client operations and, as a result, the Search Panel is capable of searching all columns in a Grid. Other areas, however, might have grids with significantly more data and be using server operations for performance reasons. A side-effect of this would mean the Search Panel is incapable of filtering on non-string fields. This not only might lead to unexpected results to an end-user, but also requires the developer to explicitly list each string field that can be searched. If a developer forgets to list only string fields, the default action will be for it to attempt to filter on all fields. If any fields happen to not be strings, you still get a loading indicator as if it's attempting to filter, but the Ajax request silently fails and returns an error 500 behind the scenes.
There are some manual workarounds discussed here, as well as some information as to why this limitation exists. It seems like the problems causing these limitations are known, as are some rough workarounds to get around it. It would be great if we could get some official support to address this limitation so developers aren't left to either work around it on their own or avoid using this feature altogether. This feature would be great if it weren't for this limitation. A single place to quickly and easily type in something to filter on, and have that filter applied against all columns could definitely save some time and be very useful, but with this limitation with a pretty technical explanation (from an end-user perspective), the unexpected mixed results could instead lead to confusion and frustration, and distrust of this feature.