I think there's 3 main issues myself and others I know have been concerned about regarding this. (this is largely based on my experiences, take what you will of this post, some of it may be entirely inccurate)
Firstly, it's the fact that good, albeit generic maps SHOULD be ranked. There are SO many BNs with a million requirements/prerequisites that some perfectly fine maps are deliberately being ignored. I feel like as a BN, all songs and maps should be considered. Sure, preferences are fine and all, but it's really sad knowing that some BNs NEVER consider anything out of their comfort zone. A good map can sometimes be swept under the rug simply because "song is too boring" or "it adds no new content to the game". Those imo are some really shit excuses. Just because a song is "too boring" for your tastes means it shouldn't be ranked? Like, are you kidding me? If the map represents the song fine, why shouldn't it NOT be ranked, other than some superficial reason? And besides, a large portion of modern content is not really new by any means anyways. Point is, when majority of BNs are THAT selective of genres, there's bound to be many good maps that are just in graved. Now, I'm not saying "BNs are lazy and bad yaddayaddayadda". I feel like although frowned upon, it's necessary. This leads to my next point.
Secondly, the BNG feels a bit understaffed. Even though I personally dislike the millions of requirements / preferences BNs expect when you request a map, I can't help but think it's not entirely their fault. In the end, they look through HUNDREDS of requests from like 10 different channels, and it's extremely straining for them. In order to cut out the already gargantuan amount of work that is held upon them, they ofc have to put in those preferences. Add into the fact that they sometimes close for months on end, and they're barely (if not) approachable at all. They do have to uphold quality standards after all (which honestly, even looking at the number of requests makes me feel sick). I feel like there should be more BNs in general to help spread out work a bit more, but ofc that comes with the problem of assessing a lot of BNs, and then the risk of possibly having the ranked section decrease in terms of mapping quality (which I wouldn't want at all). Point here is that there's just too few working too hard. Only time will tell though if there's more people willing to be in BN/NAT. Otherwise, I don't think this can be helped anytime soon.
Thirdly, there is no doubt that there's at least some preferential treatment (aka circlejerking or w/e you wanna call it).
This is what I'm not okay with. There has been cases where BNs do a Nom4Nom, or where BNs have a select few whose maps they'll always look at. I feel like this is extremely unfair to the general masses, that perhaps some people may have to work so much harder to get ranked maps than others. I get that preferential treatment exists, because the mappers involved are probably already pretty good at mapping, and in turn that allows for easy nominations. It's a win-win for both parties in this case, but that really crushes the hopes of many up-and-coming mappers. It's very discouraging to most people really, when other people have that privilege when they don't.
I definitely appreciate the efforts of the BNs, and the programmes that have come up. (for e.g mentorship etc.) Although it may be extremely hard to get into, no doubt that many have benefitted from said programme. I feel like programmes like these are a good step forward in resolving this whole "good unranked mapper cant receive attention" fiasco. I myself, had the luxury of getting feedback from BNs (even as a newbie mapper), in that I am eternally grateful for. I am writing thise because it's just sad to see my peers having a hard time getting things ranked, because of the aforementioned issues, when they are already better than some ranked mappers imo.
This was purely based on mappers that are already capable of mapping potentially rankable beatmaps. ofc this absolutely does not apply to newer mappers who more than likely don't know what they're doing and taking things too quickly. This is just purely my opinion, I do not mean any harm, nor do I want to call anybody out for this. I also would like to mention that as much as it sounds like it, I'm not labelling all BNs negatively, as I have met some really nice people (BNs and experienced mappers alike). This is overall a summary on the frustrations some of my good mapping friends have with the current system.