One of my customers has asked that I build him some wheels with ceramic coated rims for touring.

Does anyone have any feedback as to if there is any downsides to this?

For some strange reason as a racer and a WW I have never really gotten into loading a whole pile of stuff onto my bike.......

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Success is how far you you bounce back up after being knocked down

What size tyres is he planning to use? If 28's then Open Pro Cd Ceramics should work.

ceramic rims are pretty nice. They hold up really well, brake nicely in all weather and are a bit thougher than normal aluminium rims. I have used these for racing and think they are great. Only downside is that if you flat and you hit something you will likely chip the ceramic surface and make the rim unusable, but I don't think that is a legit problem when touring.
Because of the sturdyness and durability of these rims I would say they are very suitable for touring.