I've actually only met self-proclaimed "nice guys" who were actually nice guys and not the asshole version I read about on Reddit and elsewhere (though I'm sure they exist).

My "nice guy" friends are great and I feel lucky to have them as friends, and though they do suffer from some of the associated stereotypical traits, they certainly don't view women as "vending machines that spit out sex if you put in enough niceness coins" as I've see it explained. They're kind and generous to all their friends, male or female, don't feel entitled to having a girlfriend, and have never reacted badly or stopped being my friend even if I rejected them romantically, or gave it a shot (I had to initiate!) and then broke up amicably, or got into a serious relationship with someone. They're respectful, don't cross boundaries, and are all-around upstanding guys and a lot of fun to hang out with.

However, my "nice guy" friends do struggle with dating/pursuing women and tend not to be romantically assertive. A couple of them had crushes on me, but never asked me out (but then again, they also didn't expect me to magically want to become their girlfriend out of gratefulness for their friendship or get mad/upset when it didn't happen). I've also heard them lament how girls only like bad boys or hot guys and don't give nice guys a chance. When this happens I try to tell them that it's not about being hot or bad, but about confidence and taking the initiative. At this they'll say that it's easy to be confident and risk rejection when you're tall and handsome, rather than short/nerdy/fat/whatever they think their issue is, and that it's hopeless for them. When I point out the things they CAN change (clothes, fitness, attitude), they'll point out the stuff they CAN'T (height and facial features), or say that they don't want to change themselves to get a girl. Ok, that's fine, I say that there ARE girls who would like them the way they are (because they do have great personalities), but they still need to put themselves out there to meet them. At which point they go back to "nah, all girls like bad boys, even you, so why bother" and I can't convince them otherwise.

I just feel like it's this vicious cycle where they lack confidence because they feel they are unattractive, and the lack of confidence makes them less attractive (at least to me, and I imagine many others, though I'm sure it's not a turnoff for some), and I don't know how to help them. I guess it's not my job but I feel like they could be in happy relationships if they put in some effort to be more attractive and approach more women. Unfortunately, most of them are convinced the effort would just be a waste, and it's not like I can guarantee it won't be. Many of them have just kind of resigned themselves to being alone and insist that they're perfectly fine without a girlfriend... so now we just don't really talk about their dating lives too much.

So yeah, that's been my experience with the "nice guys" I've met and are my friends. Great guys and wonderful friends, definitely not entitled at all, but lacking confidence and unwilling to put themselves out there romantically. I've never had a bad experience with a self-proclaimed "nice guy" so I don't see it as a red flag like many others here do. My one bad experience was with a self-proclaimed "reformed bad boy" and I was an idiot not to run the other way immediately!