bubbagates pretty much has it right on. If the wind is straight up & down the runway, & you can actually get it to runway, then the only practical limit is the ability to get it back on the ground & keep it there once landed -- in other words when you can just do a vertical take-off & landing.

If there is a strong cross wind component, then life is much more difficult for a trainer -- the limit is reached when you run out of cross wind control authority, particularly on the ground. This will vary for different models, for example an Avistar is reasonably good in crosswinds, & a Nexstar is noticeably less capable in those conditions. The dihedral creates problems, & a model that behaves poorly in cross wind gound handling will be dramatically better when modified to remove the dihedral. In flight, the model doesn't care about the wind -- it's only the getting going & getting back part that creates problems.

The absolute limit is reached when you don't have sufficient power at full throttle to advance against the wind.