My Most Feared Unit to Play Against

So, I'm listening to my favourite youtube content creator the other day, Fritz40K. I was watching this video in particular:

https://www.youtube.com/watch?v=zW45ouQ9bWE

and while he raises a good point about the small blast templates being way less effect then their larger counterparts, it did made me think about the Thunderfire Cannon.

As of late I have been playing against a lot of marine lists. In every single list there is at least one Thunderfire Cannon (sometimes two). This particular unit just devastates my front lines. I usually play 1000 points which I laden with tonnes of cheapies (yay! cultists!). But the Thunderfire cannon is tailor made to kill my poor zealots. I don't cry about it though because that's what my cultists are there for, right? Soak up all that barrage for hopefully two rounds to keep that thing from destroying my more expensive and powerful units before they get a chance to do anything.

So here is the thing, as I go up into the higher points in preparation for the tournament do I get rid of some of my cultists in order to take more chaos space marines in rhinos? Or should I just keep my cultist blobs? I'm going to write two lists in the next couple of days to test out each scenario, and then hopefully play them out against some Imperial Fists or Iron Hands. Do you think the Thunderfire Cannon is overrated? Do you think I'm completely off the mark? Feel free to comment and tell me your thoughts, I would love to hear what you have to say.