I have no idea what Clip on the Op is using every one I have used was pretty cool.
I have two snarks and they work very well, when the battery dies they don't work.
But the battery's last longer than the 9 volts and if you forget to turn it off while it turn on by itself.
I have a boss Tu 2
it is old and had a mic built in that worked pretty good.
It may be a TU 1 it was the very first tuner made by boss, it was stepped on, many the bottom that say Chromatic are smash down inside it and it still work fine.
I also have the BOSS TU 100 which is also very old.
The snarks or any clip on seem pretty good for teaching people to bend in tune.
Again ears are best but it does help to have something to check and see you are bending flat,
if you bend sharp your ear should tell you that's really wrong or it should tell you it sharp anyway.
Tuning the guitar by ear is part of learning and should be done, Acoustic always sound better when tune by my ear than with the tuner.