D&D 5E Am I reading the Mind Flayer attack right?

[Imaculata]

Or, y'know, not do that, and let the warlock's player feel good about having a useful ability for the encounter rather than nerfing it in advance.

Well that's why I'd rather make it a choice. Rather than nerfing it directly, I'd add some consequences to it. For example, if there are healing pools, then the Warlock must be mindful in which direction he pushes the Mindflayers. He can still use the ability, but the encounters forces him to use some strategy, instead of just spamming it.

This reminds me of a 3.5 encounter I designed a few years ago, which basically did the exact opposite. Rather than adding a consequence to the players pushing the enemies, I added a consequence to the enemies pulling the players. I had them fighting on a bridge inside a cave, while giant aquatic spiders were shooting webs at them, and trying to pull them off the bridge. The players had to take cover, or they could get pulled off the bridge, into the web-filled water below.

 

[iserith]

I find it odd that some would conflate presenting a challenge to the characters with "nerfing" a class feature. Don't want to risk using repelling blast while the roots of Dzéwà are thrashing about? Call upon the fighter cut them down before you blast the mind flayer off the rogue. Or heroically take the damage to save the rogue from having his or her brain sucked out. Or if your patron is the Great Old One, give up some of your sanity to make yourself immune to the roots. This is the stuff that creates exciting, memorable stories - teamwork, sacrifice, character development. It isn't a punishment for choosing or using a particular class feature... it's an opportunity to be awesome.

 

[spectacle]

And the roots of Dzéwà that nobody has ever seen or heard of before just happen to be dangling around the one time that one of the Warlock's signature abilities would be extremely useful...

 

[iserith]

And the roots of Dzéwà that nobody has ever seen or heard of before just happen to be dangling around the one time that one of the Warlock's signature abilities would be extremely useful...

And here's where the valuable DM skill of telegraphing comes into play... you foreshadow this hazard before the PCs encounter it. The insane ramblings of a lunatic driven mad by the influence of the mind flayers include references to the pale, hungry roots of the White God, for example. When the PCs turn up to fight the mind flayers and see these horrors dangling from the ceiling, the players get that "Wait a minute..." moment and can start to make decisions to overcome the challenge. The references to the Far Realm, the Old Gods, insanity, weirdness, etc. - all solid tropes befitting the presence of mind flayers and thus perfectly reasonable inclusions in such a challenge.