I come at this from the point of view of someone who wants to breed rats that are good pet animals but also good show animals. Some varieties are easier than others and there are different reasons for it.
Some varieties have health issues, which may or may not make them less suitable for owning and selling as pets. For example – British blues based varieties are prone to having poor immune systems, topaz/buff are more likely to have bleeding/clotting disorders, chinchilla based rats if not carefully bred can produce kittens with megacolon. Most of these can be minimised through careful breeding, but it’s another selection pressure, and the more plates that need kept spinning then the more likely one of them will fall down (or that they’ll all stay up there, but wobbly and ready to collapse with little prior warning).
Most marked varieties are not particularly difficult to breed, but difficult to produce an excellent example for show (I would consider a show rat to be one that wins stars and will be a contender for the supreme challenge in a show, not one that just wins a class rosette and nothing else), and lots of the rats you keep for breeding will not be show suitable. If you’re only intent on breeding a very small number of litters then having a variety that may only give you a quality show rat every couple of years isn’t very encouraging. Some marked are easier than others – Essex tend to produce well marked animals fairly easily, and roans (although they don’t show well for long) often are nicely marked and will win as young rats at least.
Most of the AOV varieties are easy enough to breed good examples of. Things like silver fawn, agouti, cinnamon – agouti based rats tend to have a good long show life, there’s a lot of them about to find crosses of, and so they tend to do well in shows. There is a lot of competition in those classes, so you need a decent one to win, but finding the right starter rats to get to that point isn’t ever going to be difficult either.
Every variety has their own challenges – different shades to get right, nuances of how that particular colour or marking works, what you need to breed to. Realistically, it’s not rocket science, and although some are more difficult than others, the most important part is that you are breeding something you love. It’s much easier to put your heart into it if it’s something you want to do to start with, and much easier to get past the inevitable obstacles if you have more motivation to climb over them.