That isn't even something that really scares me there - alone the usual stuff isn't that easy to predict over a long term. Take for example power supply - you can't shut it all down into deep hibernation, you need some power supply that is reliable and can deliver 100% power after decades if needed.
Even something as robust and rock-solid as a RTG degrades and can fail. Or take the many analog circuits that a spacecraft needs. These age and it is technically impossible to make analog circuits that can't age. After some decades of travel, you would need essentially a full recalibration of it to make it return to normal operations. Which is pretty bad if this includes radio gear. As far as I know, they compensate the problems on Voyager on the ground by adapting the ground antennas to the aged gear of Voyager.
Or the engines - we can't even restart a old two-stroker reliable after some decades in perfect storage. Now this should work on a interstellar spacecraft?
This is all a huge can of worms, without excessive redundancy and a pretty much failure tolerant design, we can't even say that there is a realistic chance that this spacecraft might arrive at its goal. We would need many huge spacecraft like these already for making sure at least one arrives anyway.
But I think personally, any kind of FTL is more realistic as finding a design that could operate mostly flawless after 100 years without a human repairing it. Preventing a mission failure is really hard if you can only accept x system failures.
