Hello,
The original aluminum and rubber rails were tough. Eventually the rubber would harden and may cause a little noise as the chain slapped against the hard rubber. In some sever cases the rubber would wear away but the rubber would digest in the chains and gears without issue.
The new style work great for the first ten of fifteen years. The modern plastics are tough and durable when new. Eventually age, engine heat, chemicles, UV and acids in the engine oil make the plastic brittle. Now days we are spending a lot of time fishing these broken plastic chain rails out of chains, oil screens etc. In some cases, broken plastic chain rails cause timing chain failure and catastrophic engine failure.
Hopefully the plastic material is being improved as time goes on. Accelerated life testing cannot predict every condition, only time will tell.
To sum things up......don't loose any sleep of it for at least ten years. As time goes on make sure that the timing chain is not getting too loose and slapping those old brittle plastic rails. Also consider replacing the easy to get to ones as time goes on. New rails may be made of even more durable plastics in the future!
