It's got nothing to do with if the training is 'good'. It's just a different approach. It's no secret that Barca both in training and on the pitch rely more on domination with the ball than constant high intensity, a lot of their ageing players, who are funnily enough, still their best players, are a result of that. You then have someone like Dembele who has a very explosive style. When you aren't regularly engaging in that kind of activity, it will lead to an increased chances of muscular injuries as you'd be putting your body through more shock than what you are accustomed to. Dortmund played a very high intensity style as a comparison, which would have been reflected in their training and you saw both how well he played there and how little he was out.
It may well be his fault, due to lifestyle etc. but it just seems too big of a coincidence for me. Barca seem to be more a dark hole for talent than we are these days, everyone there seems to be shite and has difficulty fitting in. Even if it was down to him or bad luck etc. You've seen players with a point to prove turn around in the past, especially as they've matured. Robben is a prime example of this.
It'd be a huge risk obviously, which is why I wouldn't agree with us paying a huge fee for him, but if he was available on loan, maybe with an option to buy, I wouldn't be shocked to see him completely surprise a lot of people. I'm not so sure Barca would be completely against that either. They are in desperate need of reinforcements but are strapped for cash, getting some of his wages off the books would be interesting for them. His talent is in the top tier, he's one of those kids who are just natural ballers. Whether or not he brings it all together, is another question though.