Generally speaking, if you send an instrument-level signal into a device that needs line-level input, you will get excess noise and/or weak sound. If you send a line-level signal into a device that's meant for instrument or mic-level input, you will get distortion. Since the effects loop on most amps is designed toboth send and receive line-level signals, if you put a typical instrument-level effect pedal in the loop you will often get noise, weakness, and distortion. You may find a few exceptions though: either an amp loop that can operate at instrument level, or a pedal that can operate at line level.
They say that line level signal is much stronger than instrument level -- but I can run some line level devices off a very thin stereo cable with 1/8 inch plugs whereas instrument level needs a big thick cable with 1/4" jacks?
Not logical hence confusing.