we all know that although members of the dark brotherhood are professional assasins but they arnt gods, certain contract would be outright impossible for them, e.g.

kill a member of pijjic order
kill the thalmor leader
kill Gods (im not joking)
kill a incredibly well guarded high offical

what if after several failed attempts, perhaps results in the death of several members, would dark brotherhood abandon contract and gives refunds?

You can kill a Psijic or Thalmor leader in theory. The gods are already dead so that's easy. There are several quests already to kill incredibly well guarded high officials.

the thing is the emperor knows the darkbrotherhood and accepted his death, otherwise during the great war why cant both side just do a super well paied job to dark brother hood to assasinate their leaders? back then the dark brotherhood is still strong. There must be limitation with the guild.

things like pijjic they properbly are living demi gods in another dimenson, you cant touch them