I've not read any of Ennis' work, but I get the sense that its subversion is the kind I often dislike in superheroes - the super-dark, undermining of childhood innocence type stuff.
Miller I have read extensively, and I can tell you that while his good work shines like a beacon among comic history, he has an overwhelmingly unbalanced bibliography consisting of basically a handful of really good series and then a whole lot of shit/stuff that has dated badly (a lot of his weekly runs as a younger writer, for example, in the latter case).
Millar on the other hand, might be even worse, simply because he's 1. younger and 2. people don't seem to know what an asshole he is as much.
And I guess it's because Frank Miller seems to not know what the fuss is about and is just a relic from a previous era, a blunt, inefficient tool not made for the modern industry. He is your Grandpa. He will keep saying racist shit, so stop asking him questions.
Millar, on the other hand, seems to have a story concept of 'trigger the libs' written at the top of his notepad, underlined in red, before he starts any plot synopsis.
Hey, you like cannibals? Incest? Every single character being the worst person on Earth, including a xenophobic Captain America? Protagonists so hateful and disgusting that they commit a rape off-panel that is never addressed? Would you like these stories to then get adapted into movies somehow? Do you want every single ounce of joy or happiness in comic books to be crushed under the big, throbbing boot of toxic masculinity? Oooh! Shades of grey!
I will say Millar has one book which I like a lot, though, which is Superman: Red Son. But Elseworlds stuff is kinda cheating the system a little.
Fuck that guy.