According to the [url=http://www.bungie.net/7_Hotfix---10142014/en/News/News?aid=12283]1.0.2.2 patch notes[/url], the Vex Mythoclast received a 34% base damage nerf.
While many people felt that 34% is too harsh of a nerf, the Mythoclast actually recieved a near 50% damage nerf.
Before the patch, [url=http://i.imgur.com/7ogECuN.jpg]the weapon did 45 damage at point-blank range in the Crucible[/url] ([url=https://www.youtube.com/watch?v=uZ7U7rV47PA]source video[/url]). There are countless YouTube videos that exist that can verify this.
After the patch, [url=http://i.imgur.com/bmuwydj.jpg]the weapon does 23 damage at point-blank range in the Crucible[/url] (my image). This is verifiable in game.
This is nearly a 50% damage nerf, which is quite of a lot larger than the already controversial 34%.
I have not recorded PvE damage numbers but they seem to reflect this.
I am not giving an opinion on the nerf. I am merely presenting facts.