What does an Annipod Ultra guarantee?

Elite is a floor, not a target. The potential rating is set at the moment of capture, and the Ultra only removes the bottom of that range. A roll above Elite is still possible, so an Ultra throw is not wasted on a creature you already like the look of. What it does remove is the outcome you cannot fix later: a creature you wanted, caught at a rating you would rather not build on.

The guarantee on capture changes how you approach the fight. Weakening or stunning a wild creature raises its capture rate, and an Ultra leaves no capture rate to raise. You can throw one at a creature standing at full health and it lands. That turns a long, careful fight into a single item cost, which matters because Aniipods are consumable. Every Ultra you throw is one you no longer have.

Aniipods are throwables, so the practical limit is getting in range rather than grinding the target down.

Bosses

Only the higher tiers can capture boss Aniimo at all, and the Ultra is one of them. That is a separate property from the Elite floor and it is the reason to hold a few back rather than spending them on whatever wild creature you meet next. If a boss is the catch you want, a lower-tier pod is not a cheaper version of the same attempt. It is not an attempt.

What the Ultra does not cover

Appearance is a different guarantee entirely. Premium Aniipods such as Sparkling Cubes are what lock in a shiny glistening variant. An Ultra gives you the creature and the potential floor and says nothing about how it looks, so a shiny Elite is two different pods answering two different problems.

Not settled: How far above Elite a capture can actually roll is not settled, so there is no way yet to say how much value you give up by throwing an Ultra rather than catching a creature the ordinary way.