Maybe a CON of 0, and NO HP gain per level-up is too harsh. I'm just spitballing ideas around. How about this one as an Undead Racial Passive:
Unvitality (Passive): Gain twice as much HP per point of CON, but your maximum HP only increases by half-as much when you level up. Your HP is also not restored to full upon leveling.
This means that Undead gain -some- Max HP when leveling up automatically, but it's a lot less effective than that of living people. Also a further penalty - living creatures HP is automatically restored to full when they level up, but the Undead do not. They have to work harder to maintain their decaying bodies.
If they want to be tough, they'll have to invest points into Constitution (but if they do that, they will go farther than that of ordinary people). Their extra gains from CON represent being tougher to injure and kill because of a lack of vital organs.
Giving the Undead (and not the Lizards) the "Consume Dead" ability, which lets them eat corpses to regain HP would make for a great synergy.
