制限付きアグリゲーションリレーはどのリレーからイベントを収集してるか開示してそっちへ書き込むフローにしたら少しは解決できるかもしれないけど複雑すぎる

Yes.

Also relay hints mention 1 relay (and generally user has no idea what that relay is, so client logic of how that's chosen is critical but it's not transparent). And then there's question of longevity of relays -- so how reliable are those hints?

This is what I thought. But since most of the Devs seem to believe it's a good idea, I was wondering what I'm missing