I have no problems with the
Mercury remote, either direct, from 8-10' away, or through my remote extender (RS pyramid), which is about 20' away.
Years ago, I had problems with my first Fortec receiver's remote (Lifetime). I think that it sometimes wouldn't work when the receiver was exposed to sunshine or computer screens. At that time, I started using a universal remote that worked better, however I've since gone back to using the original remote, with no problems, so I'm not sure what changed.
As mentioned above though, some IR signals do seem to be confused by nearby computers. I'm not sure if it's caused by light from the screen or if it is electromagnetic, however I can see it happening with my pyramid extender, since it has a little light the blinks whenever it sees signals from a remote. I will often see that little light blinking when there is no remote being used, and often I need to place the pyramid where it can't see my computer screen, or remotes won't work.
I have also had problems where remotes stop working only to find that one of my dozen or so remotes had a key stuck, or the keys were being pressed because I placed one remote on top of another remote. Once I even found that a remote I had put away in a drawer was blinking away, and locking up my remote extender so that other remotes wouldn't work.