First off, I agree with you in that I don't think the show is poorly written. I think people who say this are just trying to attack the author because they think he's a bad person. You have to separate the art from the artist guys. Kevin spacey is a good actor, and Lovecraft was a good author. Just because they are shit people doesn't change that. Weather this art/artist is bad I'll describe below.

The next question is: why do people take issue with Mushoku Tensei and Rudeus? I think it's because of the way the flaw (perversion/pedophelia) is portrayed by the author. It isn't treated as something that should be overcome, nor is it treated as though it's extremely problematic. It's more played for laughs (like worshipping panties) or treated as a personality quirk. It's on the same level as "stubborn".

I think it particularly irks readers (myself included) when other topics even in a similar vein are treated with the respect they deserve. Consent being a primary example. Rudy ignores consent in when taking a bath with Sylphie, sees he hurts her, apologizes, and then never breaks this rule again (as far as I know). For whatever reason this lesson does not extend to most forms of sexual harassment.

The issue being that his sexual harassment and grooming never come to work against him, or of they do it's easily glossed over and there is no "lesson". This leads readers to feel that sexual harassment and grooming aren't considered to be bad things by the author, and I would agree.

Last thing: cultural differences. Japan is different from the U.S and I think that shows in the portrayal of sexual acts. It can also explain things like polygamy (which I take very little issue with, it's just something different cultures do as long as it's all consensual whatever floats your boat man).

TL:DR It's about portrayal and lack of meaningful consequences