My 1st trip to Hat Yai was by the coach from Golden Mile. The 2nd trip was by plane from Changi Airport. The 3rd trip was by bus from Johor Larkin Bus Terminal. I will say Hat Yai is a place to eat and shop. A stay for 2 to 4 days is enough.
I took the van from Hat Yai bus terminal 2 to Karbi.