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Yes there are a couple harder to collimate:
- a tri-schiefspiegler, you can google that;
- the Loveday folded Newtonian which uses 3 mirrors plus diagonal. You'll find this in Sky & Telescope June 1981 issue.
Refractors pose two added twists - it isn't just the tilt of the mirrors, you also have to find an optimum combination of the rotations of the elements to minimise the collective wedge error, as well as adjusting the separation of the elements. There will be a sweet spot with respect to the monochromatic aberrations as well. These added degrees of freedom make an airspaced 3 or four element refractor the most difficult, IMHO.
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