The disadvantage of fitting yourself , apart from the problems of safely working under the car, is things dont always go to plan. Things may not separate easily, needing heat or cutting , a replacement part may not fit existing pipes exactly without some 'fettling' , a part you had expected to retain gets damaged during dismantling etc. And find various rubber suspenders etc perished or missing even on sections you are not replacing.
You could have a half completed job delayed for days waiting for extra parts to be delivered , or buy more bits than you actually needed. Good savings can be made DIY and although if you were only changing a rear section it may be relative trouble free it can be a pig of a job.
