Both caused a disruption in each teams season, adding unnecessary drama and conflict. Both players also had high expectations of them, but both ended their time at the club poorly due to their interviews (yeah Lukaku’s still technically at Chelsea, but he doesn’t really feel like one anymore with him getting loaned out every season).
Ronaldos was after he had been dropped and was clearly out of the new managers plans. He was forcing his way out on a free imo.
Lukaku did his less than 3 months after joining whilst under the same manager he signed for. On top of that he did it when there was no negativity around him with supporters.