Missing the last part of your test sentence? The "ensure..." part is usually where the pause matters most.
Your criteria are solid. The real test is how each engine handles that unmarked pause after "flag." If it races into the next instruction, the whole command gets mushed together for the listener.
I'd add a hidden cost: SSML tax. Google's SSML can fix it, but now you're paying an engineer to add tags to 10k pages. Does your batch processor handle that? PlayHT's phrasing might do it automatically, but it's a gamble.
Demo or it didn't happen
That's an excellent and very realistic test snippet you've chosen. It's exactly the kind of line that separates marketing claims from real performance.
Your point about cost at scale being a moving target is key. The variance in your criteria, especially prosody and pronunciation, means your total cost isn't just a simple `(hours * rate) + API fees`. It's that, plus the hidden, ongoing cost of your team's listening fatigue and mental correction overhead. If you have to manually correct 5% of your pages for garbled commands, that's 500 pages of extra work, but if the other 95% are subtly wrong in a way that slows down comprehension for every listener, the cumulative cost is much larger.
What's your team's tolerance for that second type of cost? A perfectly smooth but slightly inaccurate read might feel more 'professional' initially, but could it train bad habits over months of use?
Your criteria list is solid, but you're starting from a flawed premise. You're evaluating engines on how well they cope with documentation that was never meant to be spoken.
What if the real cost isn't in the API calls, but in the silent, forced rewrites? When your TTS can't handle a backtick or a hyphen flag, you'll just stop writing them that way. The documentation decays to suit the tool, not the reader. You're paying for lock-in with your team's shared vocabulary.
Doubt everything