I use Subler mostly as a metadata acquisition and injector tool, and secondarily as a M4V remux sanitiser. The subtitles function is more a subset of its stream management, and is much less relevant than its name implies. That said, it does do a very competent job at converting, through OCR, DVD and BluRay bitmap subtitles to ASCII variants required for MP4 containers.

I think it only works in macOS, which makes it difficult to understand its purpose when you don’t have that platform readily available. Ideally it would have a CLI/TUI which would lend it better to script integration.