I will disclose that I am not a fan of duct work, and the ducts are a pain to maintain.
The ducts get dirty quite frequently, but if you do any project, it is a great idea to muck out the ducts; You need to hire professional HVAC duct cleaning rather than do it yourself.
You can really disinfect the inside with a disinfect brush, but then it is unquestionably easy to pop a seam or cause a crack, a small hole can create a big disaster, and losing air to a small crack means higher utility bills & possible HVAC repairs, then ductwork sealing is then required. You need to have air with sealant particles to maintenance up the holes. Another reason I don’t like HVAC duct is that I don’t feel it is unquestionably efficient! Having the whole home be one temperature just doesn’t seem smart, then why would I want a single temperature throughout? It makes sense that if you live with multiple people, you would all have your own temperature needs. Also, when cooking in the Summer you might want A/C but the people in the living room may not. If you have rooms that aren’t in use, why bother providing them with HVAC? Zone control makes sense on so numerous levels. You just can’t do that with a ducted system. Also, even when the ducts are officially sealed & slim there is still air loss. It isn’t as energy efficient as a mini split. That is why when the time comes & I need to substitute my HVAC equipment, I will choose to go ductless.