Trailer spares should ideally have a comprehensive warranty. Not all trailer spares have warranty. Those that do would actually have limited warranty. It is not just trailer spares that have a limited warranty. Your trailer too has limited warranty. There will be plenty of exclusions or exemptions wherein the manufacturers cannot be held responsible or liable and you may not get any coverage or assistance at all. Since trailer spares may or may not be manufactured by the company that also made the trailer, it is imperative you consider the warranty as it would not be in the same line as the one you had on the trailer.
Trailer spares that do have warranty will often only address manufacturing defects. Nothing but workmanship or faulty materials would be covered and hence if there is any problem of some other nature then you are on your own. Some trailer spares will have warranties that would go beyond the manufacturing defects. It is possible for some trailer parts to wear down sooner than expected and this may not be owing to any particular defect but of the nature of use. How you care for trailer spares, from cleaning to maintenance, will have a bearing. If you have not committed a mistake and no one seems to be at fault, then some manufacturers will be willing to look into the problem. This is not a given so read the warranty.
Trailer spares do not have onsite warranty. Roadside assistance is beyond any realm of expectation. You may have to pay for the postage and handling of the component to and from the manufacturer. You can always get a local technician to look into the problem and recommend an alternative if that saves you money. Postage and handling in case of larger trailer spares can be a substantial expense and it is normal for anyone to try and avoid that. Reading the warranty thoroughly will give you a lucid understanding of what you can claim and which grounds would not qualify for the enforcement of the coverage.
One of the reasons why many trailer owners tend to prefer easily available and more affordable spares is because of the lack of comprehensive warranty. Also it is not always possible to control how a particular component would react with another or the kind of fallout in specific challenging situations and hence it is often more practical to have a local company or technician take care of these needs than correspond with the manufacturer.