The same setting produces different results
Smooth skies, detailed hair or grass, and screenshots with text react differently to compression. Test representative images instead of applying one number to everything.
Judge quality at the final display size
Keeping a 4000 px source for an image displayed around 800 px can be wasteful. Resizing first and then compressing often saves much more without a visible loss.
Where compression artifacts appear first
Check facial edges, small text, dark tonal areas and smooth gradients. Smearing, blocks or halos around edges are signs that compression is too aggressive.
For the web, adjust size and format together
A practical order is resolution first, format second, quality last. Compression alone cannot compensate for an unnecessarily large source image.