Since Friar Laurence's plan goes badly astray, it can actually be easily said that the disadvantages of the plan outweigh the advantages.

One advantage to Friar Laurence's plan to fake Juliet's death is that Juliet's marriage to Romeo, and Friar Laurence's involvement, gets to remain in secret. Friar Laurence agrees to marry them in secret, hoping that the unveiling of the marriage will end the feud and unite the two families, as he tells Romeo,

In one respect I'll thy assistant be;
For this alliance may so happy prove,
To turn your households' rancour to pure love. (II.iii)

However, now that Romeo has been banished and Juliet's parents are pushing her to marry Paris, Friar Laurence's initial plan has gone awry. In order for Juliet to remain married to Romeo and not become guilty of polygamy, nor Friar Laurence guilty of creating a polygamous marriage, Juliet's marriage to Romeo must remain a secret. Hence, one advantage to Friar Laurence's plan is that it keeps the marriage in tact, while also maintaining secrecy.

However, one disadvantage is that his scheme relies heavily on effective communication. If Friar Laurence fails to relay the plan to Romeo, then the plan could go awry. As we see from the story, Friar Laurence's message does fail to reach Romeo. Instead, Friar John, to whom Friar Laurence gives the letter to be delivered to Romeo in Mantua, is quarantined for being in an ill person's home and never makes it to Mantua. This failed communication has dire consequences. Romeo learns from his man servant Balthasar that Juliet has died and believes her death is real, which leads to his own death.   

Another disadvantage to Friar Laurence's plan is that the plan relies heavily on timing. On Tuesday (possibly afternoon), Friar Laurence instructs Juliet to drink his potion on Wednesday night, allowing only a little more than 24 hours to let Romeo know of the plan. It can be said that 24 hours is a very short time to inform another person of a critical plan. As we see from the play, Friar Laurence's letter to Romeo is delayed, showing us that the critical plan was formed far too fast.

Hence the disadvantages of the need for not only thorough communication but speedy communication as well, far outweigh the advantages to Friar Laurence's plan to fake Juliet's death.