They're fine but nothing compared to a remap. Some tuning boxes are better than others - TUNIT for example has 9 variable power settings and can also be custom tuned on a dyno bringing it closer to remap like gains but you'll still be missing 10-ish bhp and 20-ish lbft of torque in comparison to tuning the ECU.
The good thing about them is they are obviously easily removed by a DIY'er and are interchangeable between cars (most models) and if that is your concern more than outright performance then it's ideal.
