Yes, they are, but you should consider a paid SSL if your site regularly hands sensitive information or payment details. However, for basic websites, a free SSL will do what you need it to.
Of course, there are unreliable and unofficial SSL certificate providers out there, which you shouldn’t trust to give you a secure SSL. However, as long as you choose an established and reliable provider – like Let’s Encrypt, Comodo, or any of the hosting providers from our list – then a free SSL can be trusted to keep your site safe.