After it has been blamed for not informing its users which protocols it uses, Zenmate says it relies on TLS 1.2 (RFC 5246) protocol with 128-bit AES-GCM data encryption, SHA256 data authentication and ECHDE RSA handshake. So from the encryption standpoint, it is good and it is even rewarded for that. The problem with Zenmate is that its free service may be more limited by time (Zenmate is a commercial company) and the plugins itself keeps running even if the browser is closed thus consuming CPU and memory resources, and remember that Zenmate is not a full VPN service but it only encrypts your Chrome sessions.
As for Browsec, it is also recommended and just as well seen as Zenmate but for the etc you used in I regularly uses VPN addons like browsec, Zenmate etc in my browser, it all depends on which plugin you are talking about: As a good safety practice, you should use a browser plugin only if really necessary. And before installing one, being informed about it (why not review its code or see if some community did that, its rating and so on) is a good thing. There are even free opensource browser plugins developed as POCs of how a plugin can compromise seriously user's privacy such as ZombieBrowserPack that can be manipulated remotely to steal authentication credentials, and even bypass two-factor authentication mechanisms such as the ones implemented by Yahoo and Google. It can also hijack your Facebook account, or get your bank account credentials if you perform an online purchase. You can imagine such a plugin purposed to fulfill this or that feature (shim) and doing something nefarious behind in the background.