It might have a good lore justification, but I think that a constant health drain just does not fit the exploration and talking-based gameplay of Original Sin 2 at all.
A different way to represent Source Addiction would be better. Hmmm... How about this, just a quick idea off the top of my head:
Source Addiction (Passive): Undead get a +1 Bonus to all attributes when their Source Points are full. When their source points are not full, Undead slowly become more vulnerable to damage over time, up to a maximum of -50%. The fewer Source Points they have, the faster their resistances tick down. They immediately lose this penalty upon acquiring a Source Point. After losing a Source Point, it takes 30 rounds/180 seconds/3 minutes before the damage vulnerability starts to take hold. (That's STARTS - so that's three minutes of normal play before the withdrawl kicks in.)
Thus, the undead are encouraged to keep Source Points, and they have a harder choice about when it's worthwhile to spend a precious Source Point, but the effects are not the same. It's not +20% which drains back down to normal, cancelling itself out.
Maybe it's me but I really dislike the idea of micro-managing every few secs/mins, depending on the circumstances and penalties. After all, there may also be areas cut off from the Source and spells doing damage to your Source points. The idea of having to reload or run back to a Source area very often if there's something off-balance, doesn't sound very appealing to me. After all, if the Source Points hit 0, they cease to exist, right?
If the devs tweak your suggestion to fit the system so you can still recover and you don't have to micro-manage like allowing players to bank Source Points in the form of potions, etc., then maybe it's doable? Or maybe we've a device that acts like a bank with interest rates.
