Those trying to defend Murk or say that he is less broken than the Valk are likely Murk users themselves.
First off, I recommended a change to the 'always go first' aspect of RI. I do believe that the problem comes from always going first, like Vek does.
I personally do not own a Murkalot, but I've used it while playing with a friend. She has all the rare pets (like Frosty <3), pets that I do not own. I might be biased for using this pet though. I think we've gotten 100 wins using Murkalot.
I however do not experience much problems fighting against Murkalot teams, I expected much worse. The thing that holds Murkalot teams back is that the pet is difficult to use. I have a great winrate against Murkalot. In nearly 60% of the cases the opponent could have won, if they used this pet better.
One must overview the whole battle and pretty much decide the best course of action. With Murkalot + AoE teams this is relatively simple, and even here people fail. With other Murkalot teams good predictions become even more vital.
I might be biased. I personally like to have some control on my teams, which gives me a good chance against Murkalot, despite using less popular pets.
The Unborn Val'Kyr is a lot easier to use. If you screw up with the Val'Kyr it most likely won't impact the outcome on battles. Mistakes with Murkalot are instantly punished.
Do I believe that Murkalot is broken? Yes.
Do I believe that Murkalot is horribly broken in the hands of a good player? Yes
Is Murkalot a problem on live servers? Not really.
I might be wrong though. I would like to see another wild pet with RI + Ghostly Bite combo. Then we can judge if this combo would impact the metagame a lot.
He is perhaps the only example of a truly "Pay to Win" pet in the game (BA/Sunlight team can be depending on what you run into, and Rag is bad but very manageable).
Do not underestimate Frosty.