Weakness: The Defensive Line
The 49ers won games in 2019 because they had one of the best pass-rushing units the NFL has seen. Dee Ford, Deforest Buckner, Arik Armstead, and Nick Bosa combined to wreak havoc on the NFL, and that continued all the way through the Super Bowl, where Bosa likely would have been the Super Bowl MVP had the 49ers won.
Statistically speaking, the 49ers defense had the same number of sacks this year compared to 2019, but the way they have come about are very different. The 49ers in 2023 have relied more on blitzes than a straight four-man rush, most because their defensive line has not created that consistent pressure that they need to change games.
Given the talent on the 49ers defensive line (they have Chase Young instead of Dee Ford and Javon Hargrave instead of Deforest Buckner), this lack of explosiveness is hard to explain. But it is the biggest separation between the 2019 49ers and the 2023 49ers.