Unfortunately this one fails in the same way most others do, and that is spelling. Shakespeare often got creative with his spelling, particularly when doing his iambic pentameter thing, and often would skip lett'rs complet'ly. So if you search all the works for "If it were fill'd" you'll correctly get Sonnet 17, but if you didn't know that and searched for "If it were filled" then you get nothing.
Failing #2 is when to use quotes and when not. Search for "to be or not to be" and the appropriate Hamlet soliloquoy pops up - but search it without the quotes and you get King John and Timon of Athens before Hamlet shows up at number 5. But on the other hand search for "is this a dagger that I see before me" with the quotes and you'll get nothing. Search it without the quotes and you'll properly see that the word is "which" I see before me, not "that". So depending on what you're searching you'll often have to try both ways.