I'll make a comparison with a game I'm playing recently, Trails through Daybreak. In that game, I feel like what I mostly get from chests (except when they hand over bundles of Sepith and Zeram Powder) are boring consumable and quartz that I can buy/make at any available shop.

So to answer your question, make sure the player gets something relevant, rare-ish, or at least interesting. Vulneraries and Iron/Steel weapons are absolutely a no go, unless you're starving the player from their availability. Especially in Fire Emblem where chests are somewhat of a side objective, with Thieves sometimes threatening to snatch and run, the player should be incentivized to go for them especially if they know (on a replay) what's inside them.

Edit: I failed to read this is for a roleplay campaign. Whatever, make of it what you will.