The actual loss of success for most voyages taking a merchant into account is closer to around 10~20% in worse case scenarios (10~20% of the success chance that is, so for example, a 80% success would perhaps drop to 64~72%), so merchants are, under almost all circumstances, are better. The only merchant that is dubious is Ardougne shopkeeper. Also, for normal goods, it is always better to go for the higher reward voyages, their average good per voyage is much higher than lower ones.