The ability to type any string of characters to make a word be pronounced in a different way and Synth V will do it’s best to spit out what you typed. Unlike another up and coming vocal synth that won’t tolerate words that don’t match up to a database engine located “somewhere out there” which is another stupid heavy handed (we’ll screw you later with a subscription) implementation.
I don’t think the said feature is actually any helpful from my perspective as an experienced Vocaloid user. I was always able to find loopholes that allow me to bend pronunciations to any word I want to input, especially using the actual phoneme mode instead of inputting similar-sounding lyrics. That’s how I was able to input Korean lyrics out of a Japanese Vocaloid. And because I’m able to use the phoneme mode to get everything to the exact pronunciations I really wanted, I would argue that sometimes the phoneme engine on Vocaloid is actually better, whereas with SynthV, even with the right phonemes I input it still doesn’t sound right. SynthV definitely has the edge when it comes to voicebank qualities though.