Is the TTS reliable?
Or is it full of mistakes? Im especially interested in the pronunciation of s and s.
Also, does Swedish have any silent letters word finally or initially?
It's far from perfect and has a rather funny melody to its pronunciation. For many sentences it gets it relatively right though.
Swedish does not have any specifically silent letters, but quite a handful of ways of spelling the sje- sound and tje- sound respectively.
S can be tricky, since our retroflex R loves to merge with the S into a tje-sound, even across a word space, which is why it'll often be heard as very similar to the english sh-sound.
Are there any specific cases you're wondering about? Or examples?