He is big .. many holes.. terrible style
When squared up to a single shooter in a non-breakaway situation, he can make a great first save and has decent rebound control.
When asked to move laterally, he drops to the ice way too easily and gets killed on the return pass.
When asked to defend against a deke, he can be beaten pretty easily, and despite his huge frame he leaves massive holes open for shooters with any real skill.
A lot of what is wrong with him can be coached out of his game... but if that was going to happen, you'd think it would have already happened by now. He's going to be 29 before the season ends. What you see is what you get - a naturally gifted athletic goalie with fantastic size who has never been able to, and probably won't ever be, able to think the game at a high enough and intuitive enough level to ever be a reliable long-term starting goalie at the NHL level.
The difference between a good backup and an average starting goalie is consistency. Nilsson doesn't have the vision to be consistent enough as a starter. He'll have just enough good starts to get you excited about what could be, with just enough bad starts to leave you frustrated about what he'll never be.