They were the strongest only to be superseded by the Senju and the Uchiha. It's stated in both the manga and databook that they were practically killed off based on fear and how dangerous their sealing jutsu and we got a taste of that through Karin's feat against Spiral Zetsu. The only reason the Uchiha and Senju weren't initially killed off was due to being apart of Konoha where they coalesced with numerous different clans whereas the Uzumaki clan for all we know, lived in complete solitude.
In defense of the Uzumaki, I really hope people refrain from claiming, "They were so powerful yet got defeated by fodder" because there is strength in numbers and such ignorance means we're contradicting very explicit details in both canonical sources in favor of our biases and interpretation of a very vague instance. We're led to believe the Uzumaki are powerful hence why the ones that are explicitly shown i.e Nagato, Karin, and Kushina have accomplished tremendous feats, far beyond what any shinobi would have been capable of.