More than one audio track
MKV is the container people use when they want to keep everything: a film with an original track and two dubs, a concert with a stereo mix and a surround mix, a recording with separate microphones. Most online converters take the first track and give you no say. If your file has several, this page lists them with their language tags and lets you choose.
Codecs you may hit
MKV does not care what is inside it, which is its strength and the reason it occasionally fails. AAC, MP3, Opus, Vorbis and FLAC all decode here. AC-3 and DTS, common on files ripped from discs, do not — the browser has no decoder for them, and shipping one would mean a large download for a small number of files. If you hit that, the page will say so rather than producing silence.
MKV in the preview player
Safari cannot play MKV at all, and Firefox only plays some of them, so the preview may be unavailable even though the conversion works. The waveform is drawn from the decoded audio rather than from the player, so you can still see and cut accurately.