As far as I know, he has never been accused of poorly training, if anything in the last year he has been praised for working hard. He has bad time keeping like many other players which is obviously not good but people make a meal out of it when it comes to Dembélé especially when someone then say that they hope that it is a smokescreen for Sancho, who has actually been accused of poorly training and is often late for training or team meetings.
And as far as I know, he doesn't have a bad attitude outside of being a poor time keeper, there is that one story where he overslept and pretended to be sick a few years ago which is probably the worst thing that he has done. Otherwise he seems to be generally liked. The main issue concerns his fitness and whether there is long term damages that have been made, earlier this year the Cope reported that according to the clinic Barcelona sent him to in Qatar, the reason behind his injuries was that the fitness programs that Barcelona gave him weren't adapted and caused relapses after relapses.
If he can stay fit, I think that he will be a very good player but he is clearly immature and can't be considered like someone that you leave on his own, he is typically the type of player that needs to be checked by a veteran player, that is also true for Sancho.