I have been booked in Category I cabins in the past and other than being lower in the ship, exactly the same as the staterooms above it. Of course, it also depends on where it is located. They are less comfortable in the forward portion of the ship (at least in their Sovereign Class).
We have Category H cabins...which I assume are identical to Category I but one level up (3rd).
Can you share what made this class "uncomfortable"? Was it ship movement, noise, rocking or just plain small size?
There is a bit more movement and noise in the cabins situated at the very bow of the ship, but my comment about being a bit more uncomfortable was in specific reference to Sovereign class ships. The cabins are configured in such a way that some of the cabin space is taken up by a window ledge - the closer to the bow of the ship - the deeper the ledge. This is not the case on their newer ships.