I suppose it depends what you consider efficient: I would counter that using a mere 26 letters to encode all the varying sounds of English is wonderfully parsimonious, an incredibly efficient use of those characters. Such an efficient encoding does however, as you point out, make decoding more cumbersome, as it requires memorisation of the specific pronunciations of strings of letters up to and including whole words. In that sense, however, it is very similar to the Japanese (ab)use of kanji, which - as I pointed out at the very top of this thread - has the same problem. For a given kanji, you need to see it in context to be able to have a reasonable chance of pronouncing it correctly (and sometimes even that isn’t enough).

What I’m slightly puzzled by is your apparent confusion as to what a syllabary is: as I gently tried to hint in my reply (and someone else has now more explicitly pointed out), hiragana and katakana are syllabaries; kanji is not, even if it is occasionally used that way (当て字). I’m not sure to what extent that undermines what you were trying to say.

But, to engage with the substance of your point on the efficiency of Japanese syllabaries, we first have to put aside the fact that they retain two distinct systems to encode the same sounds (a baroque inefficiency surely without peer in any other language). It is true that modern kana allow for efficient decoding - there is almost no ambiguity in the sounds, は for ha/wa excepted. That reliable decoding does, however, impose a fairly hard limit on the number of sounds they can express, so I am not sure what you mean when you say “[y]ou can use them to encode anything as well”.