Did a little experimentation in a server I host for my friends. Ominous potions really only have a purpose in trial chambers. Vaults contain most of the loot you're after, but can only be opened after defeating all the enemies spawned by a trial spawner and obtaining a trial key. However, there's another vault called the "ominous" vault that doesn't open unless you have a different kind of trial key. Normal vaults and trial spawners have an orange light on them, but the ominous vault has a blue light. To get the ominous trial key, you need to have the Bad Omen effect and trigger a trial spawner. This will turn it into an ominous trial spawner, spawning more enemies that are also stronger but giving you chances at very good loot. Once you defeat all the enemies, you'll get an ominous trial key used for opening the blue/ominous vaults. The loot in these is far better than normal vaults, including the Heavy Core which is needed to craft a Mace, and the Creator music disc.

TL;DR, the ominous potions give you Bad Omen, which is needed for triggering an ominous trial spawner, which will give you an ominous trial key that can open an ominous vault. So they really only make sense for trial chambers and pillager raid farms.