The Mythoclast was quite the mission to get my hands on, so I was excited to get it when it dropped. I spent two days doing bounties to get it fully upgraded and noticed a few things. The first thing I noticed is that you can't hold very much reserve ammo for the weapon, something around 140 extra rounds. With the upgrade, the magazine goes from 21 rounds to 54 rounds, but that only really compounds the problem. Another thing, on that subject, it may just be psychological, but it feels like I am finding less primary ammo when I'm using the Mythoclast, it could be I notice it more because I am constantly having to look for ammo due to the low amount of reserve ammo.
On the upside, it does help me finish up the Grimoire card for fusion rifle kills, which may not matter for some people, but I'm a bit of a grimoire collector.
Now, in PVP, this is where it gets the most attention because of it's apparent strength. However, I can say with confidence that it is pretty much on par with SUROS Regime in terms of actual strength. There doesn't seem to be any reason to bring it into PVP if you already have Regime. Yes, it outperforms pretty much all other guns, save for maybe Timur's Lash if you have good aim and upgrades, but it's definitely not what I'd expect from the hardest weapon to obtain. Thankfully, the reload speed is fairly quick, and with PVP, primary ammo isn't much of a worry, so that knocks off one of it's downfalls.
In closing, I like the gun, but I don't love it. It just doesn't make me feel like I'm holding the game's ultimate weapon.