Super happy user of EnvKey here - I'd be interested to hear what sets Doppler apart.
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.
I'd also argue that EnvKey might be a bit too absolutist about security, in that we think the user experience greatly suffers as a result. We have a different tradeoff that emphasizes secure defaults and best practices while also allowing for necessary features like audit logs and syncing with different infra providers. We spent quite a bit of time considering the tradeoffs of zero-trust, but our user experience would suffer as a result, and so we have taken a different approach.
That said, a Content Security Policy doesn't actually address the issue I'm raising, because an attacker with server access could simply remove or modify it.
Of course, no system is invulnerable to any attack. But in practice, Doppler's architecture implies a much larger degree of trust (any server breach = secrets compromised) than EnvKey's (servers can be fully breached and secrets still aren't compromised). Doppler looks like a great product in many other ways, but I do think it's important for users to fully understand the risks they're taking.