Why are thrust reversers so unreliable?
It's not so much that thrust reversers (TRs) are more unreliable than other components of the plane. However, they have a very important impact on the airplane's performance. Hydraulics, flaps, and even engines fail regularly. But engines and hydraulics can be backed up by redundant systems, and an airplane can still land without flaps. But engineers learned from Lauda Air Flight 004 that an airplane may not be controllable if the TRs deploy in flight.

So in the interest of safety, the TRs have additional precautions. The reasons for not aborting a landing after they are deployed are discussed in your first linked question. Reliability is also not the only reason for having no TRs on the A380 outboard engines, as discussed in the second linked question.

So to answer your question, there is indeed a technical challenge in making the TRs reliable: they absolutely must not deploy in flight. This means many safeguards are put in place, and they are designed to fail by not deploying if there is any doubt about whether it is safe. A TR that won't deploy is more desirable than a TR that deploys when you don't want it to.