Not trying to sound disputatious (sorry -- I respect you!), but you are confusing two things here: accuracy and precision.
The accuracy is the ability to get the value right. In the case of a tuner, this would be the string's frequency. A tuner that is accurate to +/- 2 cents will report the frequency of the note to within about 2 one-hundredths of a semitone. In marksmanship, an accurate shot is one that hits near the aiming point -- presumably, the center of the target.
The precision is the ability to report the value very reproducibly. That is, it is a measure of the variation in the numbers reported. In marksmanship, this would correspond to the size of a grouping of shots.
You can be very precise but inaccurate. In marksmanship, imagine a tight cluster of shots, but centered systematically at some point well away from the bull's-eye.
To set a mandolin bridge, you do not require any particular level of accuracy. None at all, in fact! You're simply comparing two notes to see if they're the same or not (the harmonic and the noted 12th fret). What you require for this activity is precision. In fact, you can do this comparison entirely by ear, and without any tuner, if you like. And if you do it with a tuner, all you care is that the tuner registers in (nearly) the exact same way every time it 'hears' a note of that frequency. Not whether it gets the frequency right.
The accuracy of a Snark or NS Micro tuner may be +/- a few cents. But their precision -- which is all you need to set a bridge -- is much better than that. It's typically around 1 cent or better, in fact. Very, very few people can hear better than that. In fact, we usually try to cheat when comparing two notes that are nominally the same, by hitting them both and listening for beats (at the frequency difference). But when listening for a 12th fretted note and its harmonic on the same string, you do not have the option of playing them both at once: you have to play them in succession.
When you strike a string with a pick (particularly an older string), the note usually starts out a tiny bit sharp and then goes flat as it settles. That change can easily amount to 1 or 2 cents, and you easily can see it happening on a Snark or NS Micro tuner -- because the tuner has a precision (not accuracy) of 1 cent or less.
The bottom line is that a Snark or NS Micro is perfectly capable of being used to position a bridge, due to its excellent precision (not its less-than-ideal accuracy).
Also, the changes in accuracy that naturally develop due to less-than-deal intonation, due to (1) imperfect compensation by the staggered slots in the bridge saddle, (2) the stretching of the string as a result of fretting it, and (3) string age, nonlinearity, and other imperfections, all amount to MORE deviations than you will ever get from using a Snark or NS Micro to set your bridge. And that's not even considering the many-cents 'inaccuracies' that arise from the equal temperament placing of frets on the fretboard. Don't forget that a major third interval in 12TET is nearly 14 cents "off" what it ought to be a just temperament! (And the tritone is off by a whopping 17 cents!)
Those would-be perfectionists among you who think you may be getting things 'right' by attempting to use a (costly) strobe tuner to place the bridge position within 1 or 2 cents of the its ideal location have lost sight of the forest for the trees, in my view. You will likely never get all 8 strings to within several cents of being perfect at the 12th fret, due to things like imperfect saddle compensation and string stretch during fretting. And you will never tune the mandolin perfectly, either.
Finally, as pops1 very usefully pointed out, you cannot expect to optimize the bridge placement for all possible notes. If you spend all your time fretting down the neck, the best compromise is different from where the bridge would go if you hope to play more up the neck.
Like so much in fretted instruments, bridge placement is a compromise. Please don't overthink it. Let's not forget that the OP wanted to know about Polytune vs Unitune vs the Peterson strobe for setting his bridge. My answer is that they all will work just fine for that purpose! Instead, my advice would be to buy the tuner that you prefer for most occasions while tuning. 99% of the time, you're using your tuner to tune strings, and not to locate the bridge. Get the tuner you prefer for daily tuning, I say.
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