Well in the 21 it is the same as with the F-15 for example. It's a bit harder to lock the target because I can't slew the TDC with the ECM filter on. On the other hand, as I said it is way more visible on my radar display, so it's easier to find in the first place.
In the Migage itself I find it pretty hard to lock a jamming target, because the contact jumps around the display all the time. There probably is a burn through distance, but I haven't figured it out yet.
As for the FC3 arcraft, sorry I can't say as I don't fly them. If I would have to guess it's probably the same effect as with all the other jammers in the game.