[Phanx]

That's an awful lot of unnecessary event checking.

You don't need to listen for PLAYER_DEAD and PLAYER_ALIVE at all, as UNIT_HEALTH will fire for the player unit at those times anyway. You don't need UNIT_ENTERING_VEHICLE and UNIT_EXITING_VEHICLE, as you only care when those transitions end, not when they begin.

ht:SetScript("OnEvent", function(self, event, unit)
    if UnitIsGhost("player") then
        return htt:SetText("|cffb40000Dead")
    elseif UnitIsDead("player") then
        return htt:SetText("|cffb40000Ghost")
    end
 
    local hcur, hmax = UnitHealth(unit or "player"), UnitHealthMax(unit or "player")
 
    if hcur >= 1000000 then
        hcur = format("%.1fm", hcur / 1000000)
    elseif hcur >= 1000 then
        hcur = format("%.0fk", hcur / 1000)
    end
 
    if hcur == hmax then
        htt:SetFormattedText("|cff00b400%s", hcur)
    else
        htt:SetFormattedText("|cffbf0000%s", hcur)
    end
end)

I'm fairly sure, but not 100% sure, that UNIT_HEALTH will fire anyway when you enter or leave a vehicle, so you don't actually need to register for those events. However, if that's not the case, just register ENTERED and EXITED to tell you when those transitions end, and adjust the event handler as follows:

+	if event == "UNIT_ENTERED_VEHICLE" then
+		unit = "vehicle"
+	end

	local hcur, hmax = UnitHealth(unit or "player"), UnitHealthMax(unit or "player")