Comfort is actually used for something, but it's not obvious. It's a hidden value and it's not really "fleshed out". Comfortable objects give a special buff when used, and uncomfortable objects can get a negative buff when used, but a vast majority of items are "neither".
You're correct about the hygiene and bladder values- it's about speed of increase. For showers, higher hygiene = faster increase. Upgrades can increase the speed further.
I've looked deep into these things because I actually have two mods I've made myself for these things- Comfort Motive, which restores the Sims 1 + 2 style full scale motive for Comfort that prevents Sims from standing around all day and forces them to relax, and More Money More Motives, which makes lower value items provide even slower motives than usual and higher value items provide even faster motives than usual.
The latter was partly made because it bothered me that some low value items had high motive values and vice versa...so I just across-the-board (by item type) multiplied their rates based on item value using a rarely-used EA function that does just that!
It's just another problem that necessitates a true pricing overhaul.