Most unique items that have special mods on them have multiple underlying stats that are hidden which are represented by a single “display affix” which is what shows up on the tooltip. There are very few unique items, if any, that actually show all of their affixes like normal.
The affixes that are filterable are defined as individual affixes in another affix list. This means that they can be collectively added to a big list in the filter UI.
The way that unique items are serialized also throws another spanner in the works. I hope we can find a way to bring better functionality to it in the future but it would likely require a significant rework of how unique items work all together.