Aggressive marketing tactics and a deletion trap hidden in the fine print
I highly recommend avoiding SimplyBook.it if you plan on using their advertised "free" plan long-term. While they lure you in with the promise of a free version, they set up a bureaucratic trap for the user right after the trial period ends.
Once your trial expires, you are bombarded with emails. If you assume—like any reasonable person would—that "it's a free account, so I don't need to do anything to keep it," you've already lost. Hidden deep within the text and postscripts of their emails is a requirement to actively click a link to confirm that you want to continue using the free version. If you ignore these marketing emails because you are busy running your actual business, they will ruthlessly delete your entire, meticulously set-up account after a short period without a real final warning.
It is completely unprofessional and anti-consumer to make the survival of a free account dependent on whether a user clicks a hidden link inside a flood of sales pitches. All the time spent setting everything up is gone, and so is the data—simply because you chose to ignore their aggressive nudges to upgrade to a paid plan.
If you are looking for a reliable tool that just does its job in the background without making you constantly worry about your account being wiped, look for a more user-friendly alternative.
Verdict: Terrible user experience and unnecessary stress for small businesses. Not recommended.