Did they push too hard (which became a standard in later sets), then they were Thin prints (these Thin prints will be mentioned later in this article)
It's a shame the base creature costs so much mana, but you can flicker it to build your board and win with the waterbending ability, alongside white's ample token support like Mondrak, Glory Dominus
As the market shifted toward Alternate Art and Trainer Gallery cards, the Nessa Rainbow Rare dropped off a cliff
While this isn't guaranteed, there are plenty of cards that have done nothing but increase in value as time has gone on
Prioritize synergy over rarity : A winning deck is built on how well your cards work together, not just how rare they are
Pip-Boy 3000: A powerful modal equipment, Pip-Boy 300 will often net you extra mana on attack, but sometimes it might grow a creature or loot a card for you