What are the marginal benefits (and costs) of running a Helm over ProtonMail service? I can see none.

The justified concerns the security community has with ProtonMail is: Crypto in the browser is bad (mitigable with Qubes), and How do I know PM isn't serving me a backdoored JS.

IIRC, Helm has auto updated binaries so backdoor-free code isn't a delta. The best I can come up with is: server+CPU observation/isolation is stronger on local hardware relative to PM at the cost of network observation. Hypothetical: "Ok a Spectre-style attack is out, spam emails and let's do some timing-correlation traffic analysis"

I actually own a Helm, and I like it very much!