What are the advantages of "Arduino compatible" boards
While Arduino is an open source system, "Arduino" is a trademarked name and may only legally be used for products from the Arduino's originators or licensed suppliers. Arduino are quite clear about this requirement - see here - Logo and here Trademark 
and here What makes an Arduino board an Arduino? and FAQ

As a result, compatible microcontroller boards are legally required to NOT identify themselves as being "Arduinos", so the term "Arduino compatible" is about the most that even an Arduino-identical clone can claim.  



As karan notes, some Chinese sourced Arduino compatibles "have issues".
however, I have had no problems at all with Arduino compatibles manufactured and supplied by this Chinese supplier. As well as the basic devices they sell various interface boards (shields) and related equipment. Prices are astoundingly low, free postage is included in many cases (about 2 to 3 weeks delivery time to New Zealand) or you can pay for courier delivery. I buy Nano or Pro-Mini compatibles devices  - usually in batches of 10. As you will see -  Nano compatible costs $US3 each in 10's. Pro Mini compatible is currently $US1.68 each in 10's. !!!
Uno compatible here is $3.80 in 10s.  The voltage requlator may be a different type than than on 'real' Arduinos (but work OK) and versions with a USB-serial bridge IC use a non FTDI IC and you need to source and load the correct (freely available) drivers. 

NB: (1) I have NO commercial connection with this company except as a very happy customer.
(2) I'd expect you to have no problems with these or other products from them but I of course have  no responsibility for what they sell.