But yeah, I would say Defy Death probably needs some kind of minor nerf.
Or the rules for Instant Death do (How to play, pp. 22-23). If Instant Death was set not at the character's hit point total but at (IMO) the much more erasable cap of the character's Constitution score, then the ability remains powerful, but is not godlike.
It also keeps the Instant Death rule in play across the levels, and not only at low levels.
Win, win.