Why do adjacent kitchen outlets need to be on separate circuits?
You guessed it. You put adjacent receptacles in a kitchen where you regularly have high wattage items, toaster, blenders, microwaves, on separate circuits.  Additionally, each circuit in a kitchen or bathroom should be protected by a GFIC outlet.

I'm not sure of the code in your area, but typically you use 14/3 (the thinner wire) only for lights on a 15 Amp breaker. For receptacles, you typically use 12/3 and a 20 Amp breaker.

If you have a failure, it won't be catastrophic as long as you match your breaker amperage to your wire capacity. Though if you are using toasters and stuff on your island, you may be popping the circuit now and then.