CAT IIIa capable Head-up Guidance System was certified in 1977. but there was no takers for about twenty years until Bombardier and Dassault figured out it would be cheaper to give their steed CAT IIIa capability by installing it than developing autoland for F2000/CRJ/DHC-8. In my four winters on Q400 I used ıt ın anger about 50-odd times, always successfully, which was just normal around my outfıt - no one ever had problems with adjusting to manual cat III. One just sets up everything properly and when GS captures it's just matter of keeping guidance circles perfectly concentric and speed error bar as short as possible. Everything else is monitored by F/O, head down. Provided one can see something at 50ft, flare and rollout guidance help lead to stop in predictable manner. There is also takeoff guidance that enabled us to reduce minimum takeoff RVR from 125 to 75m.
Interestingly, I used to fly A320 with CAT IIIB NO DH capability, currently fly 738 with CAT IIIA and both of them are restricted to autoland only below 200DH/550RVR.