I don’t think you need to worry that much about bot attacks.
Unbounce uses a variety of publicly known and other unknown methods to keep the bots away.
In my experience from hundreds of pages and hundred of thousands of leads, bot attacks are not even in the top 20 of things I worry about.
However, if you would feel more at ease a 3rd party check is one way to do it.
I would avoid CAPTCHAs since they are a pain to setup with a native Unbounce form and could potentially decrease your conversion rate.