You probably got me wrong a little. The thing about trusting BNs or mappers is that some BNs do it and that should be punished, nominating it without even checking is insensible, but people do it, because they believe it's good when BN mapped or bubbled it.
What I mean by "It's just about how perfect do you require the map to be" means that even if you're a newbie to certain mode, you have your own mind to make questions like "Is that what should be in a rhythm game? Does that make sense with music? Does the spread scale consistently?" and basic questions which apply to every mode. Even if you don't mod the mode often, you may increase your requirements on nomination if you're unsure about the mode, if you are strict enough, you'll see when map looks perfect to you and that's not difficult to determine in any mode. Obviously, someone who never modded taiko won't be able to mod those Inner Oni maps with 1/8s etc., but if you never modded taiko, are you able to determine whether Kantan to Muzukashii are perfect or complete shit? Of course you are. You have Taiko RC, you may check them, it actually doesn't take that much to understand and remember. That will prevent you from nominating the most major issues. Those "obvious mistakes for this mode because you don't know it" are usually not so hard to determine and if you're just a little bit unsure, you may skip the map or point it out in your mod. That's just my opinion, I don't force it to anyone, but I think if you just set very high requirements, you'll be able to know what doesn't and what does pass through qualification.
"It's not only about the quality." - That of course depends on what do you consider quality to be. For example I count quality as: Not breaking any RC, is clearly sensible with music by hitsounds, note placement, volume and SV changes, no patterns that could be misinterpreted for other patterns, no patterns that could be misleading in any way. For other modes, you'll most likely not know what are the "confusing patterns" and so on, so you'll just point out what you think it may be. By simply pointing out everything you can think of, you cannot do a mistake, worst scenario, you'll know that certain thing is okay for the next time, but even if pointing out the most minor things, you're just making sure everything is alright and if you do that, I'd be surprised if you get maps disqualified for really important reasons. Learning to play certain mode doesn't take that long time and you can master to Hard difficulty like within one week, then you should be able to determine what is uncomfortable to you and you can simply point it out.
Now I am not saying that everyone should be nominating every mode, absolutely no, but shouldn't be restricted to. The whole point is that if you're a BN, you're allowed to nominate everything, that encourages you to mod every mode and it's just upon your decision whether you want to nominate or not. If you feel like you sympathy with the map, give it in-depth look and if you think it's flawless, why not nominating it? I just don't believe that there are people who don't notice major flaws if looking properly, even if they're not great at that mode.
My point is, make overview that says the primary mode, but don't stick us pins like "Taiko Nominator, CtB Nominator", because that discourages from even modding other modes, while you would be still able to nominate good maps that you can be perfectly sure about. If you're mainly focused on osu!, you'll still be able to say that ENH mapset of CtB is worth or isn't worth nomination, because it's logic is almost exactly the same as osu!. Taiko and Mania will require more thinking, but you'll still be able to make it by logic and if you feel any single thing is not what you expect it to be, don't nominate.
Obviously, nominating first map you mod on that mode is obviously a risk you shouldn't ever take, once you make like 30 mods where you mention everything, you should be able to do that. You'll watch what was changed in future and what led it to qualification. As a BN, you know how to mod your mode, so learning another one is just analyzing and applying the same logic as you do for your mode.
Edit: Probably opinion based, so I'll just leave this here as mine, and only mine opinion and eventually will discuss once we're really supposed to discuss about this problem. I think I said everything, so if there's some misunderstanding, I'll rather appreciate PMs. You also all know I usually make drama, so I better watch myself and keep it out of forums unless I really have something brilliant to say. This is not attempt to make any drama even though it maybe feels controversial to some of you.