I am sometimes lucky to see a few free remaining submissions on their accelerator the last time I used it was last week but right now I tried every hour always zero free submissions. I believe someone uses bots to automatically submit unconfirmed transactions.

Viabtc Captcha is very basic and easy to bypass they should replace the reCaptcha with Google reCAPTCHA v3 to get rid of these bots or else no real users can use their free accelerator.