My tortoises of all species and ages get dandelions, mixed with other weeds 3-4 times a week from May through September. I've never seen any negative results from it.
I offer fresh picked dandelion green and flowers just about every day to the CDTs here....along with their grazing and other occasional romaine....large red leaf lettuce....
This year due to the dryer winter my normal dandelion meadow is not so great...so not able to offer as much as usually do....
If dandelion is the only green you offer then I would not do that as the only green....but if part of a variety of food offerings I feel is a beneficial food....also, the torts here are way hydrated so that is another factor I take into consideration....
As part of a healthy mix of greens I think they're great! And they're free so how can you beat that??? My RT loves em and always makes a beeline for them when we take her outside.