Even if all you care about is security, I don't think Site Isolation makes a real-world difference right now, as it's simply a defense-in-depth Spectre mitigation in case the first-line Spectre mitigations, which are deployed in both Chrome and Firefox, fail. At present, using Site Isolation to protect against compromised renderers is a TODO for both Chrome and Firefox. It's true that Chrome is closer to getting there, though.