why fishing rod will almost always disappoint you (in slayer the spire 2)
TLDR: It is not bad. But it upgrades the wrong cards, has no immediate benefit, and you could pick instead other strong relics that help you survive the first act.
Slay the Spire 2 is a video game where you climb a spire to kill the evil architect at the top. Each floor is filled with enemies, regrettable decisions, events, and bosses. You start with a base deck of simple attacks and defends. With each enemy you defeat, you are offered to add one of three new (different!) card to your deck - these card additions are the main way to modify your deck and give your character identity.
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Fishing rod is a relic - relic are items that offers you a small (sometimes huge) bonus, independent from the cards you play.
Fishing Rod - "Every 3 normal combats, Upgrade a random card in your Deck."
At a glance, this is pure upside without any compromise obtained at the perfect moment. Upgrades are small but permanent bonus added to a specific card. Strike, once upgraded, deals 9 instead of 6 damages. Over the card's lifetime, that's dozens of damages done. Fishing rod also happens to be a relic obtained at the beginning of the game - thus, the advantage it offers accumulates early. And upgrading cards the normal way is usually traded off against the chance to rest and heal - fishing rod gives you more reason to rest than to greed in another upgrade which you will inevitably regret as you health diminishes to 0 immediately after.
The title is not to claim that fishing rod is bad - it is decidedly decent. But there are stronger relics you could obtain early that frontloads more of its benefit - most people/runs don't survive the first act, hence the importance of trading off maximum potential to immediate benefit. In addition, because your starter cards are so weak, even when upgraded, they are usually strictly worse versions of cards you could obtain from combats (and because the upgrade is random, you are likely to upgrade a starter card as they make up most of your deck). The upgrades mean nothing if you don't play it.
So yeah, fishing rod is not a great as you think. But it is not a bad relic - more ways to play the game is good!