There's nothing WRONG with using ballads for tone practice, but there are two ways in which it's not as good as specific tone practice.
1) Very few ballads cover every single note in the entire key range of the horn from low Bb to as high as you can play altissimo. No ballads involve the complete range of dynamics from pppp to ffff unless of course you specifically do something about this. Every note, and every dynamic level, is a little different in its response.
2) No matter how you play with the time, you'll still be constrained by the song's form to moving from one note to the next to keep the song going. So if you need to go back to that low F#, you probably won't.
My particular tone practice consists of playing each note on the saxophone from low Bb to the highest altissimo I have, starting at pppp, crescending to fffff, then back down to pppp. If the note drops out when very soft, or breaks up when very loud, I go back and try it again. On the lower notes of baritone, I'll need to take each note in a couple of breaths. One of the objectives is to increase dynamic range. If you are regularly practicing going from too soft to too loud and back again, while trying to maintain a constant tone quality and a constant pitch, over time the soft and loud dynamic levels at which you can play with a pleasing tone in tune will get softer and louder. The wider the range of "just barely in control", the wider the range of "in pretty good control" will be, and the wider the range of "command of the instrument with complete control of expressive qualities" will be. (This applies to range as well; someone who's spent several hundred hours concentrating on starting low Bb at pppp level, or playing high F in the palm keys at ffff without having the reed close up, willl have a far better control of these extremes of tessitura.)
And the last point: I recommend that tone exercises be done, whenever possible, outdoors in a location far away from reflective surfaces. This helps build a big husky tone with vibrancy. Avoid practice room tone.