Foundry VTT user here. I do not know about Roll20, but it must be similar.
I see 2 problems with importing generated map.
1. The hexes do not start "flush" at the edge. In Foundry, that means that I need to manually offset the map to fit the hex grid. This is a manual process; Foundry has a tool that helps quite a lot, but it's time consuming and imperfect. Ideally, the exported map from PS would have hexes starting flush at the edges. Here are the 2 ways "flat top" hexes cap be flush:
2. The "grid size" is not specified. In Foundry, maps are defined by how many pixels fit in a single grid square (the devs suggest at least 100 px, and that is what I use). For maps using a grid (like the ones generated by OPDG), it makes things super easy: export at 100 px, use the same size in Foundry and boom, everything lines up. In the best world, exporting a map from PS would be just as easy. Ideally, PS would scale their hexes to pixels per square so that exporting at say 100 px per square would fit easily and perfectly on a VTT. I read a reddit post where someone describe how to calculate it; maybe that could be useful to you: https://www.reddit.com/r/FoundryVTT/comments/t8anvc/hex_grids_in_foundry/
Thanks for taking the time to read and answer the comments! Also, thanks for everything you do :D