Update: my ETH finally got credited. I sent the deposit from Bitstamp on 26th January and it was eventually credited on 28th Feb. Talking to Huobi support it seems they manually reconcile all smart contract deposits at the end of the following month in which the deposit was made.
Lesson: DO NOT deposit ETH directly to your Huobi account from another exchange account. Instead, convert to LTC or XRP and send it that way.
The problem is not Huobi, is the way Bitstamp sends the ETH, which is different from all the other exchanges. Anyway I don't understand why you should change ETH into XRP or LTC, and maybe in the meantime the price changes: you simply send them to your MEW wallet and then to Huobi, you pay only a few cents and 0 risks.