The Great Cipher used by Louis XIV remained unbroken for 200 years. What were the factors that led to such a secure cipher?
The cipher was pioneered by a father and son duo, most of the specifics to how it exactly worked were known best by these two people. With the death of both father and son, the specifics of the cipher were quickly lost. When there are no people around that know how to use and reproduce the cipher, the motive to crack it is lost. There was some important information enciphered with The Great Cipher, however nobody was actively using it, so resources used to crack ciphers would be diverted to cracking ciphers used at that time. The lack of motive is the smallest reason as to why it took so long to crack; the cipher itself is very elegant and complex. This cipher was not one that took a written word then simply changed letters, it was a completely new way to write down the language. Languages operate with distinct sounds that can be represented by letters, putting two letters together will change the sound. Writing a cipher with syllables in mind will make it more difficult to crack, especially to a cryptanalyst who writes with an alphabetic language (like English). On top of that, The Great Cipher had certain traps put into place that would make certain parts look like gibberish causing cryptanalyst to reevaluate the type of cipher.
In my opinion, a syllabary cipher would be most effective today. This is because most of society is literate and thinks in a similar manner to the way we write, letter for letter, not letter for sound. Using syllables, but re-vamping it with more traps, would confuse people because they are not used to naturally thinking in that manner when writing.
(I double checked some facts here http://en.wikipedia.org/wiki/English_orthography and here http://en.wikipedia.org/wiki/Syllabary)