I thought Merlin had got it wrong when it identified an osprey somewhere I did not expect to find one, but that was later corroborated by other sources (there was in fact an osprey pair nesting nearby).
It does sometimes get it wrong, obviously, but because it knows your location, it does put less likely identifications with a red dot (I think - it's a while since I've had one) to show they may be dubious.
I would certainly never rely on an app to identify mushrooms or anything edible, but getting a strong steer on what bird you can hear singing from the middle of a bush is not exactly a life-or-death matter, and it helps you learn to recognise the calls yourself.
My London garden this week has had blue tits, great tits, sparrows, goldfinches, robins, jays, collared doves, pigeons and ring-necked parakeets (not a mistake).