FFmpeg options for maximum compatibility?

[jason0x21]

I've got a bunch of video files in various formats (mostly in mkv containers), and I'm having a terrible time converting these files with ffmpeg to get them into something react-player will play without giving me the message "The media resource indicated by the src attribute or assigned media provider object was not suitable", which is not terribly helpful.

I've been trying the following, which the mozilla docs that are linked to from here at least claim is compatible...

ffmpeg -hwaccel cuda -i input.mkv -acodec aac -vcodec hevc_nvenc output.mp4

...but I'm still getting the message. Is there an assured set of options that's going to work with this player?