Curious to see this subject brought up. I did ask on
some aspects of writing Fakémon a few weeks ago, but that was the "internal" facet of writing them compared to your "external" facet, as you ask: what do people think of them?
Writing Fakémon is much like writing original species in fiction from my perspective, meaning by writing them you have to skirt well outside the grounds of fanfiction and the stable support of shortcuts like "the readership knows what an Arcanine is already" and you have to worry not only about how to design a Fakémon well, but also how to
write about it well. Since this would in general terms tend to impact how approachable a story would look like - too heavy worded, too worldbuildingy, not enough action, too navel-gazing, etc- I presume that is part of why you'd see such a response to the idea of a fic with Fakémon.
On the other hand, the
same people who say that kind of thing about how Fakémon can not possibly work, have no issue with their own OCs in writing lined with stuff that seriously needs some writing description and development, such as sparkly hair, eyes deep as the core of the universe, attractiveness that turns entire governments around, a backstory about how they have been over the course of their life
all of a cook, a spearman, a news anchor, a cheerleader, a scientist, a local town superhero, captain of the kingsguard, wanted pirate and (in)famous quarterback on the way to the nationals before they took an arrow to the knee, and then also happen to have special mind powers triggered because their parents are DEAD!!!. But they do they.
If your story has Fakémon, good for you, but you need to treat them with the proper understanding that, in writing, they are not established Pokémon, and you need to also treat your audience wit the proper understanding that they will be seeing "Pokémon" that they don't know. other than that, a story with Fakémon is a story
like any other (in particular, it's no different than say a Star Wars or My Hero Academia story with their own OC species / variants).