There's certainly room for alternatives in this space! I'd say the major difference from my perspective is that EnvKey uses client-side end-to-end encryption and a signed desktop application instead of a web app interface, giving it quite a different security and trust model than Doppler.
Because Doppler is delivered as a web app, its users are implicitly trusting Doppler's servers on every request. If their servers were compromised, user data would be at risk despite any tokenization or encryption they might be using on the back end, because the attacker could simply inject malicious javascript into the html of the initial web app request. EnvKey's architecture doesn't allow this.
