I personally think you should feel free to post that type of feedback.
First off because in my opinion any feedback is better than no feedback. Better to have one person tell you which points they liked and which killed it for them than to just have everyone ignore the thing you put out altogether.
Second, because if you also mention which parts you did like, it can give someone a nice boost in confidence to know that even if this particular person has their reasons for not playing the whole thing, at least they enjoyed parts of it and cared enough to let you know about it.
Third, there’s a chance that your opinion is shared by more people than you realize who wouldn’t necessarily say it, but who would like your post to show agreement. I imagine it would be very useful to an author to know where their story loses a decent chunk of potential readers. Maybe they can change that part, or find a compromise in order to reel those readers back in. Even if they can’t, at least now they know why they don’t have as many people reading their work, as opposed to maybe assuming that their entire work is just horrible enough that people don’t even care to comment at all.
And finally, while I agree with the points about action-ability, I also think that as a reader you can’t always know for certain what is and isn’t actionable to that particular author. One author might be dead-set on their MC being over 40 years old, while another isn’t necessarily opposed to letting the MC be younger. For the second author, maybe a bunch of people would assume they aren’t willing to change that thing, and so they turn away from the story. Meanwhile, the author might just assume that nobody has any issues with this particular thing because nobody pointed it out. In that scenario, the author would lose out on potential readers while those readers miss out on a story they could have enjoyed, if only they’d pointed out their issues with it.
You won’t know for sure unless you point it out, and if the author has a good reason for the story or the MC to be the way they are, who knows, maybe they can convince you and others like you to give them a chance anyway. And if they just won’t budge even if they don’t have a good reason for it, so long as you’re respectful and you accept their decision to do with their story what they want and leave them to do their thing afterwards, I really don’t see any harm in it.
But that’s just my opinion, of course.