UPDATE below on 1/9/2020:
Yongnuo replied to me that indeed, all non-warranty products need to be sent back to CHINA! They have "the seller" handle in-warranty issues, which, considering NO warranty service exists in the USA, the only options would be an exchange or refund.
Their website does state (you have to hunt of it!) they have the right to exchange for a REFURBISHED replacement, ...meaning your replacement under warranty might have needed the same or a worse repair than what you are returning to the seller. This is a huge gamble for any buyer. Just stay away from this company until they finally (if ever) have a USA based repair representative established. Right now, they are only based in China. you've been warned!!!
I received a $495 lens from Amazon, a 14mm Yongnuo lens in Nikon mount. I realized that it came with no warranty card. I inquired to Amazon and first was told Yongnuo handles all warranty repairs, and then challenged the Amazon rep that NO Yongnuo repair center is on record in the USA and the Yongnuo website states that the seller handles warranty repairs.
He then changed his tune and said, yes, Amazon would handle all repair issues. (the rep was totally clueless of course) But, upon more research, Yongnuo, based in China, seems to be selling direct from there to USA retailers. In an extensive search on-line, no where is there any record or mention of a repair center based in the USA, meaning, you are liely going to deal with this company outside of the USA which means, they may not be held to requirements like a USA distributor would be (Sigma, Tamron, Tokina, etc).
AVOID Yongnuo at all costs. You are basically buying an international/gray market product not warrantied in the USA. No seller is going to repair a Yongnuo product. and the most a seller could do is refund you your purchase price.
I returned my 14mm Yongnuo the same day it arrived and got a full refund from Amazon.
I then ordered a much more expensive 14mm lens made by Sigma that comes with a 6 year warranty and is fully covered in the USA!
Spend the extra money on a name-brand product where a USA distributor (for the manufacturer) stands behind it 100% with a full warranty and all repairs are done in the USA. Do not buy gray market, or any international product, because you basically are buying it as-is with NO warranty! Yongnuo seems to fit this scenario.
You do get what you pay for and although Yongnuo products are much cheaper compared to the competition, you'll only get what you paid for and some misery along the way if your Yongnuo product ever fails.