There are several issues in ballet dancing which may cause some problems for dancers. Things like weight management issues, damaged feet, and torn ligaments (or worse) are the perennial issue of ballet dancers, in particular. That being said, the positives (as listed above) are great benefits for anyone who chooses to participate in this activity. It's beautiful and meaningful, a winning combination in my book!
I would say it depends. As far as health concerns go, you do worse than taking ballet. It helps burns calories, exercises muscles, tone, firm and all that. I have not personally tried, as there no good places to practice in my area (and with my knees, forget it. It's a younger person thing), but I have seen it elsewhere.
If it's dancing, well, I personally lean towards waltzing myself, so I may be biased. It is good dancing, so if that is what you like, then go ahead.
Perhaps Shakespeare gave the best advice on what is good or bad as Hamlet tells his former friends Rosencrantz and Guildenstein,
...for there is nothing either good or bad, but thinking makes it so... (II,ii,241-242)
And, Friar Lawrence of "Romeo and Juliet" remarks on the qualities of good and bad as he reflects upon the danger of carrying something to excess:
These violent delights have violent ends,/And in their triumph die, like fire and powder/....The sweetest honey/Is loathsome in his own deliciousness....(II,vi,,9-13) For naught so vile that on the earth doth live,/But to the earth some special good doth give;/Nor aught so good but, strained from that fair use,/Revolts from true birth, stumbling on abuse./Virtue itself turns vice, being misapplied (II,iii,11-15)
Any artistic expression is good as long as it is not "misapplied" and it does not exceed reasonable bounds. For, whenever the artist becomes obsessed with practicing and succeeding to the detriment of one's health, friends, family, etc. the benefits of the artistic activity become "misapplied" and are no longer virtuous. Certainly, ballet is a demanding discipline, but even with its occupation, one must balance (no pun intended!) his/her other values and not neglect them or the ones who love him/her.