We can not say any of the activity is difficult or easy, everyone has their own choice my personal opinion is on page needs more focus or a experienced guy as it can not be changed more frequently and the other hand off page can be hand over to the freshers or newbies.
According to my way, both on-page SEO or off-page SEO strategy is no more difficult. Because it depends on people workability and capability of processing. So different people use a different strategy for doing on-page or off-page SEO.
On Page SEO is the main things for SEO. If you publish quality content and internal link on your post, then it will help you to Rank your keyword 85%. And then 15% need to off page SEO. SSC Result 2019