Basic image formats are supported: jpg, png, psd, tga, bmp, and gif.

HDR images are now supported in Image Editor Plus 1.5+.

It might take a long time to process a high resolution texture (4Kx4K or higher).

Tiled image (UDIM) is currently not supported.

We know undo operation does not work in some situation. Unfortunately, currently we cannot handle undo operations including image access (need to call some Blender internal functions), so it seems to be hard to fix the issue. Some other add-ons in the market have similar issue.

No. Pasted Layer is a temporary layer, so you have to merge pasted layers to paint on them.
Also, keeping pasted layers in a Blend file is not recommended. These layers or their link data may be lost or accidentally deleted.