That is true. Trident is overall harder to find compared to Eternity.
1. It is UU, so not many Arena decks utilize it. Eternity is used in Arena decks.
2. Scorpio and Neptune brings 1 Poseidon each, but Destiny and Lionheart have 2 Eternities, and Osiris have 1.
3. Eternity is more preferable to Trident during the 500 score quest.