Nothing dubious, the unit from USA so warranty is from USA. No difference if I were to buy from Amazon US and ship to SG myself. Just whether you have the stomach to buy non local units.
I have bought many phones from overseas before. Even the rare cases that it becomes faulty, it usually fails after 1 year which would made no difference if I had bought it locally.
The more important thing is whether you can get it repaired out of warranty locally. The local service center may not have the correct parts for your model even if you are willing to pay for it.
My 2 cents, if price with gst almost the same with local, just buy local set lah...