I use the chainsaw when the mortise happens on a round face. I did and English Tie Join frame where we left the inside face of the posts natural, just peeled the bark. The brace mortises up to the tiebeam were on the round face. Plunge cut into either side of the mortise, I think I may have used some red marker on the chainsaw bar to help guide the depth of cut... Plunge cutting is not recommended for the inexperienced user!
The photo below shows the frame, but not the round side of the posts.
