Garret Wang was frequently late on set and was even suspended from a few episodes. He was suspended halfway through season 3 and was going to be released from his contract. He was supposed to die at the end of Scorpion part 1. But then he was in People's 50 most beautiful people so they kept him on.
as far as the set being unhappy, from all the conventions I've been to this is not true. They were always joking and playing pranks on each other. They even still meet a few times a year for dinner together.