I remember Yuna's interview when she decided to come back. She said after she came back to Korea and have been training w/ Korean skaters and coaches, she's thought she could do competitions more if she keeps training in Korea. Training out of Korea was so tough in mental-wise to Yuna and she felt lonely as her life was just full of only training and a dull monochrome life. She really wanted to be at home and be with her family and friends.
Now she can enjoy her training more with Korean skaters/coaches and spend her time with her family and friends after training/on weekend. So psychically, it's very helpful to Yuna I believe and training in Korea highly motivated her to come back to competitions. This also seems obvious when she interviewd with Kurt Browning in CBC at last Worlds.
So Yuna needed someone who help mentally rather than technically to compete (but I think she improved her tech too since she back to Korea ) and her current coaches have been doing great job.