Ahh, this again...
The problem here, really, is that we simply
don't know, and every viable solution has unattractive drawbacks:
If we leave everything at Unknown, the vast majority of Undertale pages become unusable in debates, and they do nothing to help standard wiki visitors get a firm handle on the general ballpark of their strength. That's no good.
If we go through with Aiden's proposal, then sure, they might be that powerful, but we have no solid reasoning to actually explain these ratings on the pages themselves. Subjective opinion statements are fine to have, but they do not a stat block make.
We could simply stop using Check Stat scaling and go by feats alone, but that leaves most all of the verse at Unknown again, and they scale to each other regardless so the point would be rendered moot.
If we were to linearly scale stats presicely, using 2-B UtU and Low 7-C Tsunderplane (which I still don't agree with but hey, what did I literally just say about opinions), then we'd have stuff like "Everyone in Undertale with 60-something and up ATK/DEF is potentially up to 2-C, since they're closer stat-wise to Chara than Tsunderplane". Similar logic is applied to
Mettato NEO even now, actually. ...I really hope I don't have to explain why "hurr durr all top tiers are Universal because reasons" is a bad idea, and I don't just mean how Base Undyne harming Asgore (30 ATK/DEF difference) would suddenly become wildly infeasible.
Same for just brushing all bullet board feats off as mere abstractions and unmeasurable; sure, everyone wouldn't break Tier 8 and all the downplayers would be happy, but that's so contradictory to our standards and just generally stupid that it isn't going to happen.
The lesson to be learned here, ladies and gents, is that Undertale really just
isn't conducive to Versus Debating as practiced on sites like ours. Between the wild inconsistencies, vagueness of combat, quirky sense of humor, lack of fourth wall, pinning down a tier for them is nigh impossible. I'm not 100% comfortable with what we have now, but it's better than any real alternative: surely, I don't need to explain why straight-up deleting them isn't going to happen in a thousand years. Really, there's not much of a real solution to be had, and that's that. :/