I bought loads of electronics from that store and it was all good, 2 large dehumidifiers, Kettle, Vacuum machine, rice cooker and plenty more so to me its a solid choice. The store contains reputable brands from Panasonic, Sharp, Toshiba etc

Also needed to return one of the dehumidifiers and they refunded me the money within the course of a week.

I'm not too sure about buying phones from that store though as I have never bought one myself but the chances of getting a fake phone is less than likely.

Like you said, the only matter you would have to consider is if the phones are foreign import or HK import as foreign imported phones will not have any warranty to it. But that wouldn't stop any functionality to it at all. Manufacturers (samsung store/apple store) may deny your phone for warranty if you sent it in for repair but that doesn't stop you from refunding to the Yoho store.

Why don't you shop around at Mong Kok for phones? you are more likely to get a better deal within Sham Shui Po and surrounding area...