How good is Prometheus on Marth? I always hear people say he's an amazing dragon but he's never suggested for any builds
He's not particularly amazing in the way that you would absolutely choose to use him if possible. It's more that he (and his type of aura in general) makes high dragons more accessible, for ranged characters in particular. The strength + resistance split is much better for an attacker than the regular hybrid strength/hp split, and he only needs a couple unbinds to get the stats necessary to be helpful. But builds are probably going to use (MUB) 5* pure strength dragons instead, especially considering fire has 2 of them, because why would you pick less strength if you already have enough HP to survive?
For a practical example... for HMS, if I'm not playing as Verica, my alt is Sinoa. Before the 2 print update, to have Sinoa survive as long as there was a Vanessa in the party (with me having decently high dojos, maxed print, 5.3.0 weapon, promoted to 5* and 45 mana, etc.) I needed only 1 UB Prometheus. I would have much preferred to use my MUB Ifrit for like +200 strength, but as there was no full fire facility that wasn't possible without a Vanessa that had maxed her co-abilities.
Now with 2 prints (I use a MUB copy of the 3* print Bellathorna as her second), that's eased up a decent anount. She can survive using Ifrit with any Vanessa, but I still need Prometheus for when none are in the party. That's not uncommon, so he's still useful to have around for that.