My first impression when entering websites with such "hero animations" is noticing my gpu usage spiking and my power consumption increasing by 20W because somebody thought it was cool to have some pointless but "cool" canvas/WebGL/CSS (or whatever it is) animation.
What's the point of having the GPU if you never use it, right?
I want to use my compute resources for stuff that are actually useful/relavant. I have no problem if somebody makes some animation that actually conveys something relevant for the topic of the website/product/article etc, eg some animated plot to showcase some data, esp when that can convey more than a classical visualisation. To be fair, I also dislike hero images in general, as they are distracting/useless, consuming bandwidth resources and screen space (esp in mobile). But "hero animations" take this a step further.