I can't make any general statements. I checked both tools against a document that was recently corrected by a human proofreader. Out of six errors found by the human, Grammarly identified three and proselint none.

"only half of the mass if accounted for by the stars", misspelling "if" -> "is"

"in the heath death scenario", misspelling "heath" -> "heat"

"The quantity follows Gaussian distribution.", missing article