Yes and no.
You can not use FretMaestro to level frets on a scalloped neck, you would use a sanding beam, and then use FretMaestro to radius and crown the frets by touch, which is much better than using straight files... you'll get the radius and the crown at the same time and they will be Symmetrical Vs the Asymmetrical result straight files make.
Otherwise, take it in for a PLEK.
Prior to 1944 fretboards were hand carved, and as such they were not truly symmetrical.
From about 1944 to 1965 fretboards were made with manual feed machines similar to a wood shop shaper... these were and are true symmetrical fretboards.
In 1965 the shapers were upgraded to auto feed. Faster production, but not a quality upgrade.
In the mid 80' is when the first CNC machines came in; 1987 seems to be the for sure year, and by 1992 the big makers had all changed over to CNC.
So the odds are that only pre 1944 vintage guitars need to have the fretboard trued before FretMaestro can be used for the level... that said, FretMaestro can still be used for the radius and crown work.
Fretboards that were allowed to be over humidified or dried out will not be symmetrical. They've either been shrunk by dehydration, or expanded from too much humidity.
If frets are being pushed up from the fret slots, that's a pretty good sign that the neck got too humid.
If the Frets are loose and lifting up the board got dried out.
In either of these cases there are two remedies as I see it:
1. Take it in for a PLEK on the fretboard.
2. Replace the neck, not too expensive for bolt ons, much more cost and trouble if not.
The 3rd choice is to pay a shop to hand plane the fretboard - this is guesswork, it can be close, but far from perfect. And it will cost a lot more than the PLEK and more than a new neck.
Beware of the repair here... it is likely that the fretboard has also delaminated in areas away from the neck, string tension will make sure that it will continue to creep that way, how long is anyone's guess before real separation becomes apparent.
This means that the only real solution is to replace the neck, or the fretboard, entirely.
Guitar makers started using shapers in 1944. Shapers use a blade shaped to the desired fretboard radius and cuts a perfect radius shape, no choice, the shaper cutter is shaped that way in hardened steel.
In 1965 the Shaper way became automated instead of hand feed.
Then from 1985/7 through 1992 all major makers switched to CNC.
Now... look to see if there are cracks or slight splits in the fretboard, if so, it has dried out from improper care and is likely pushing up a few frets from the slots.
If the wood grain is coarse, it has likely been exposed to prolonged humidity, the coarseness results from excessive moisture opening the grain.
Asymmetrical fretboards are very rare since the mid 1940's.
Poor care in humidity control over a prolonged period of time can and will cause the wood to shrink or expand, in which case the Symmetrical shape will be lost.
In such cases. I recommend either replace the neck, or correcting it with a PLEK, however, since the wood has been corrupted there is no guarantee that reshaping the fretboard to symmetrical will work. For one thing, it is probable that there is some delamination between the fretboard and the neck that will only get worse over time because of string tension.
Absolutely.
Try making a symmetrical radius using a straight file. It cannot be done. It can be close as in "Close Enough" which is always the case especially when correcting errors means having to take off even more fret material which then means having to go back to the frets that were already done to try and get them to match... but who is going to do that? No one when it means many frets, instead, "Close Enough" is the rule, not the exception to the rule.
If you do not match the dial settings then the depth of cut will be cantered... that is user error, not FretMaestro error.
If the fretboard is a mess, meaning really Asymmetrical and/or twisted, you should not use FretMaestro for the fret level. But you can use it for radiusing and crowning with far better results than using a straight file.
Yes, by far.
What would take the old school way six or seven hours to get done FretMaestro can do in less than an hour. In some case as fast as 15 minutes on for example new frets, or a new guitar that you want to be very sure that the frets are symmetrical so you get the best possible guitar setup, hence playability and intonation.
Basically, None.
Follow the instructions, and everything will be smooth sailing.