Tune a guitar - how hard can that be?
Actually, harder than you might think. There is a big difference between a guitar that is acceptably in tune for casual playing, and a guitar that is as perfectly in tune as it needs to be for recording.
If a guitar, acoustic or electric, is even slightly out of tune, then it will clash with other guitars and - even worse - with electronic instruments, which are of course always absolutely perfectly in tune.
Oh, you've got a digital tuner, have you?
Well don't expect that to work. Yes, it will help you tune the open strings perfectly. But will the guitar still be in tune for fretted notes and chords? And if one chord sounds good, will they all sound good? Probably not.
The problem is that the frets of the guitar will be very slightly inaccurately positioned. Also, the nut of most guitars is fractionally higher than it needs to be. This can be greatly improved with a little attention from a skilled guitar technician.
Perhaps the best way to tune a guitar is to tune the chord that is the root chord of the key of the song so that is digitally correct. Beyond that, you need to realize that tuning is a subjective thing and cannot be entirely determined by measurements, no matter how accurate they might seem to be.