Apart from the initial teething problems during the opening of T5, I wasn’t aware of any major issues. I’m sure someone will be quick enough to correct me if I’m wrong.
I make the connection from T5 to T3 on a regular basis from Germany to the Far East and Australia and have never had any problems. Through check in all the way! Defeats the object of booking with a full network carrier if you collect at T5. As with any airline, the chances of your luggage not turning up at T5 is just as high if you check them all the way through, and if that does happen, both you and your luggage run the risk of missing your onward flight!