Placing ultimate expressy filter laoders gives you a normal ultimate express loader instead. Also, you cannot "p"ick the Ultimate Express loader. Slower tier miniloaders work fine, I haven't yet researched higher tiers.

Also, the miniloaders in general seem to be quite buggy - often I have to remove them and place them again to make them work, this most often happens when placing the belt after the loader.

There is no "Ultimate Express" tier of miniloaders. Are you referring to the Bob's-compatible "Ultimate miniloader," the Ultimate Belts-compatible "Ultimate miniloader," one of the other Ultimate Belts-compatible tiers, or something else?

Do you have a save game that reproducibly fails to work when you place a miniloader? For the most part they have been quite reliable.