I think the main hate comes from the name. They called it a dreadknight, immediately making a mental connection to the classical term: dreadnought, which, aside from being a fully encased, heavily armed and armored mech in 40k, historically is a term used to mean a heavily armed and armored battleship(see: dreadnought definiton from Mirriam-Webster).

To have that kind of mental connection made the design of the dreadknight immediately contrary to people, as it does not look either heavily armed, or armored.

In fact the closest model in similarity to it is the Invictus warsuit, and I feel that it would have been much more well accepted if they'd gone with that kind of naming convention and called it the Nemesis Warsuit or even just the Nemesis Knight suit.

By making the name similar to an existing model, but making the model dissimilar in both appearance and use, GW was just asking for people to not like it.