I feel pitchers in general are undervalued compared to hitters. In the T206 era, while Wajo, Matty, and Young (and Plank besides T206) are no doubt expensive, they don't compare to anywhere to Cobb, Wagner and Jackson. Also, I feel like the likes of Lajoie and Speaker have risen to near the same cost/value as the aforementioned pitchers.
I have not collected the younger cards of the 1950's-1960s for years, but aside from the Leaf Paige and Koufax rookie, I believe the same can be said of Feller, Spahn, Whitey Ford, Bob Lemon, Early Wynn, Robin Roberts.
But I think this has always been the case and unless there is major shift in the way the hobby views pitchers vs hitters than I imagine it will stay that way.
As far as the question asked, I will answer with what I know - pre WWII and post 1900. I think all postcards are undervalued as well as 1920’s exhibits, especially Ruth and Gehrig (these are the earliest Gehrig’s). I think pre-rookies are undervalued, like DiMaggio PCL, but there are not many (BN Ruth!), I think cabinets are undervalued, and I think all early non-major league issues (except t206), and especially Zeenut, are undervalued. Basically, things that are not “cards” per se, or not major league cards.