Atlas (and Felt, as another commenter mentioned) are interesting, but seem to be targeting a market I feel is too small. If you're established in GIS, you likely have your own stack (and biases). If you're new to it and need something more than just a visualization tool, these offerings can work well, but I fear that users may quickly outgrow the functionality and then move to a more conventional GIS offering.

I do like how Felt includes QGIS integrations as a marketing point. I feel like tools like these are great compliments, but not wholesale replacements. "The new standard for GIS software" is a gross overstatement. If I normally deal with data in the multi-GB range, limiting my uploads to 250MB seems woefully insufficient.

And I think far too many people have experienced data loss when any new platform goes under. Both Atlas and Felt have "sign in with Google" but not Microsoft OAuth which just seems odd to me unless they truly aren't targeting the existing desktop GIS user community. Import/export from OneDrive/O365 would likely be a selling point to many GIS users.