I think there are a few things that make Purdy a target for doubters. You could use any of these as evidence either way in a rational discussion. These are what people are saying, right or wrong.
1 - He fits perfectly into the scheme he's playing in so it's hard to tell how much he's managing vs the team putting him in the right position to be successful.
2 - They have a lot of offensive weapons that can bail him out.
3 - He's had a ton of missed picks over the 8 games and people still count those against him.
4 - He doesn't have the biggest arm.
5 - He doesn't have the physical traits or play style you would typically associate with a star QB.
In the end, he's managing the game very well, making some good reads, and doing enough to keep their offense moving. He's never going to look like the most natural QB and he'll have some games where he probably doesn't look like he belongs, but he did a lot last night against 87% blitz rate to show he can still be successful even under a ton of pressure.