Free SSL comes with some limitations. If you are storing Credit Card details on your website/server, or if you have high volume transection, then I would suggest go for paid SSL such as DigiCert.
Yes it is absolutely safe to use Free SSL/TLS Certificates.
The SSL certificate verifies the legality of the server of the website, encrypts data sent out, protects data integrity at some stage in transmission and decrypts the information sent out once it lands at the secure server. The certificate is vital for online stores since it offers clients assurance that their information is kept secure and safe from online hackers and identity theft. In advance of set up the certificate, it is vital to choose the kind of server the website is handling. In some cases, websites employ Apache servers necessitating an Apache SSL certificate.