If we are comparing directly to Calamity there are a lot of other things that you didn't mention. Calamity hits one turn earlier, for starters. (9 resource up, play calamity the following turn, Martyr gets dropped turn 10, cannot detonate until turn 11). You almost always need to detonate Martyr WITH priority - meaning the opponent can do things like play Call the Crusade or Scouting Mission the turn you detonate it. Playing it off priority means it can more easily be canceled, say by Word of Command + Death Ray, Humble, Timely Strike, Evict, etc.
Even if you can land the Martyr with no issues it can still be CTA'd, Kidnapped, things like Resolute Initiative can be played, etc, which makes the AoE detonation a very unreliable form of board wipe. The only benefit of Martyr that is reasonable for most games is it triggers morale loss, which, while relevant, can often be triggered in other simpler ways. (Say, Mass Death, Yuanshi's Wrath, Desolation, etc.) On turn 10 if you are playing a board wipe, more often than not, you will lose, or be critically close to losing, if the board wipe should fail. Martyr has a huge element of unnecessary risk associated with it that simply is not present with Calamity.
All in all I believe Calamity is a better general purpose card that fills the same niche as Martyr of Death. Even in the decks that would want to run Martyr, I believe they would also still prioritize Calamity over it, and there is not enough room in a well built deck to have both of them filling the same role simultaneously.
tldr; Calamity is better.