For what it's worth, which isn't much because this is probably outdated: I remember trying grist a few years ago and leaving mildly unimpressed with form support (I think because I was hoping to have image upload in the forms and that wasn't supported yet).
Since it is relevant here: support for uploads was code written by a French contributor, and reviewed by a developer working for the French gov (ANCT/DINUM) and a developer working for Grist Labs. Grist Labs has since maintained and improved on it. The forms feature itself was inspired by an integration built by Camille Legeron at ANCT.