For what it's worth, I think BDSup2Sub-created files currently have two player compatibility problems.

First, the BD subs work fine on all hardware players I've tested and on MPC, but flash for an instant and disappear on VLC. I've found that you can "fix" BD-SUP files created by BDSup2Sub by running them through BDSupEdit (just open and re-save the file, no changes). The resulting file then works fine on both MPC and VLC, and I suspect it will also work on a hardware player although I haven't tested that yet.

Second, the DVD subs wiggle around on some hardware players (still not sure what the criteria are for which ones do this). I've found that if you use BDSup2Sub to create a SUB/IDX file, and then use SubToSup to create a SUP file, the resulting SUP file is different than the SUP file that would have been created directly from BDSup2Sub (binary compare shows they are different). How different? Fixed? That part I don't know yet, I'm still looking for good test hardware. I thought maybe Doctor M could try it out.

Anyway, I'm pretty reliant on BDSup2Sub for a lot of heavy lifting on my project, so finding a simple process to fix these issues without having to completely replace my existing processes is appealing to me.