Given the number of daylilies it would be hard to generalize, and it may depend on the pest and disease and the specific cultivar. Not all cultivars are necessarily equally resistant/susceptible to all pests and diseases. Although daylily rust would have originated on diploids because tets don't occur in nature, and the species in Asia still get it badly, there's been some suggestion that tets might, in general, be more resistant. On the other hand, in my garden more dips seem to get spring sickness than tets. Is that just because of the specific cultivars in my garden? I don't know and with so many thousands of different daylilies out there few of us have the same mix.
It's common for individual plant cultivars or species to differ in their resistance to various things, one might be resistant to a and c but not b, another might be resistant to c and b but not a, for example. As long as what you have is resistant to what occurs in your garden, that's what counts unless you are hybridizing and want to distribute plants elsewhere. Even then you can only test for what you have there unless you send them out for testing, same as southern hybridizers can't test for cold hardiness in their own gardens and northern hybridizers can't test for heat tolerance in their own gardens.
Well that was a long rambling response - can you tell I need an excuse not to get on with the housework
