The engine has a timing chain to drive the two camshafts and a balance chain to drive the two balance shafts.  If the balance chain or its drive sprocket (gear) fails, the engine performance won't be affected, but it will not run as smoothly.  The nastiest side effect is that of worn balance chain/sprocket parts falling into the oil pump or lodging in other rotating works - in either case, you could be looking at a ruined engine.  On only the 91 and 92 models, the lower balance chain sprocket (and only that sprocket) was not sufficiently hardened and has a definite tendency to wear.  Picture.  You can almost certainly count on that sprocket to fail within 200 K miles on the 91 and 92 models, and probably much closer to 150 K miles.  It may fail silently and you may only notice a slight increase in engine vibration.  OR, it is possible for the sprocket failure to wipe out the oil pump or to jam up the whole works and cause catastrophic engine failure.  You just never know.

The reason the listed repair cost is so expensive is that A) getting to the sprocket requires quite a bit of hard work and B) it's usually best to go ahead and replace the timing chain, tensioner and guides, as well as the balance chain and sprocket, once you're in there.  Many people combine this work with a head gasket replacement.  Lots of work; lots of money if you're paying someone to do it.

Subsequent years and replacement sprockets are sufficiently hardened and do not have this problem.

Symptoms: Increased engine vibration (best case).  Total engine mechanical failure (worst case).

Preventive maintenance: Use the best oil possible and keep it and the filter changed regularly.  Really, though, replacement is the only way to prevent this on the 91 and 92 models.  There is no easy way to inspect for wear.