Sadly, it does. Plug into a Line6 Helix at my feet (and then into guitar amp or PA), I'd say that it loses 10% of the gain, or even more. Like having the preamp gain dropping from 6/10 down to 4/10.
Is it annoying? When you run out of battery during a rehearsal or a gig, and suddenly the feedback screams, yes it is annoying.
Should it prevent you from buying it? To me, not at all! I won't go back to having a cable wrapped around my legs, thank you. Besides, this thing is tiny and weighs nothing. And the battery lasts longer than the declared 5 hrs. Sure, there's no way you will be warned when it wears off (and it does so abruptly), but I can't think of a way it could.
In conclusion, I will happily use this thingy until I find something of the same size that does not lose signal, and does not cost an arm and a leg...