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There have been cases of timing chain guide failure. Statistically few in the grand scheme. Plastic chain guides are used in many engines.
Failure normally starts with a bad tensioner, which can be heard as excess chain noise. The slop on the chain breaks the plastic guide, which either causes skipped timing and/or seized oil pump.
Guidance is to keep up the oil changes (this maintains both the lubricity on the guide and healthy tensioner).
The guide was redesigned for 2015+ n20's. There has been no conclusive evidence the redesign addresses this particular "issue". Could simply be new part supplier.
To recap - statistically low failure rates. Good potential to avoid.
On the carbon buildup issue,
Walnut blasting is an effective, expensive maintenance for carbon buildup on valves.
Seafoam (injected into the intake) is a good lower cost, simpler alternative.
Some say an oil catch can on the pcv line will mitigate carbon buildup. It might.
I wouldn't regret the N20 engine. The normal engine noise may sound very unrefined, and almost diesel-like, but there are many reliable high-milage examples around.
Happy Driving!
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