I'm thinking mostly of how easy it is to defeat him. This has been a thing with Gillen writing Sinister back to his time on Uncanny during the Utopia years. A gunshot to the head killed him, whereas in earlier times he'd just laugh that off. Or in the issue of Immortal where he kept trying to kill the council, and he was killed by Hope borrowing Storm's powers and zapping him with a lightning bolt. Between his molecular control over his own body, his healing factor, his enhanced durability from Thunderbird's X-Gene, and his telekinetic shields, there's no way that should be able to even hurt him, let alone kill him.