Even those are not "true-temperament" instruments like a piano, they adapt a "well-temperament" tuning system like Bach's keyboard tuning [0]. The end result is closer to true-temperament in some keys, and farther in others.

Oh, and that applies to standard tuning only. YMMV with alternative tunings, especially the open tunings.

0: https://www.guyguitars.com/truetemperament/eng/tt_techdetail...