Option A: Reanimation is Temporary
Just because you reanimate the body does not mean it is any less dead. After a major battle, you may find yourself low on man power; so, you start picking up corpses and throwing them into the cauldron to bring them back, but this does not make them alive, it makes them undead. I can think of two good ways to handle temporary reanimation: either the body continues to decompose after being brought back until it becomes useless, or the magic simply fades over time until the body just runs out of magic, then does a super rapid decomposition like vampires that turn to dust when you stake them. Either way, this will make them quite useless as permanent soldiers or slaves.
Because of this, it is never ideal to kill a loyal soldier or peasant. A living person might have another 2-3 decades of usefulness to you; so, as long as you trust them, it's better to keep them alive. On the other-hand, if a village full of peasants decide to rebel against you, and you need to kill them anyway, you might as well put them in the pot and get an extra growing season out of them.
The biggest consequence these pots would have on warfare is that routing would be a VERY bad outcome for a battle. If you engage an army and kill 90% of thier men but fail to take the battle field, then the next time you fight that general, he will have all of his men back plus everyone you lost too. This might be motivation enough that if you are going into an uncertain battle, that someone might occasionally choose to kill thier own army to make darn good and sure that they will not run away before the battle is truly won. Doing so always comes with the pretty big price of knowing that your whole army is lost in the long run, but in situations where short term gains are sure to determine the course of a war, it could be worth it.
That said, I think the most common tactic that would emerge here would be to form undead penal battalions. Instead of sacrificing living soldiers to the undead, when a war starts, you empty your prisons and ghettos of undesirables, and turn them into undead fooder. If this is a pre-modern army, your battle formations would probably look like a Roman Republic legion where the role of Hastati and Triarii are replaced with undead, and the living troops serve the role of Principii. This is because you would want the bulk of your undead in the front lines (Hastati) because they are the most expendable. Then you put in your living troops. Not putting them in the front line means that you improve thier moral, and keep them fresh for then the battle really comes to a head. Then the back few lines are more undead to kill any living troops who try to retreat and because they would hold the line best when things get so bad that ad Triarios redisse.
Option B: Cerridwen decides who is born again
Cerridwen was not just the goddess of rebirth, she was also the goddess of death. Just throwing bodies into the pot does not ensure that she will grant you favor by bringing that person back, instead she only brings back those who are servants of death: soldiers, executioners, murderers, etc. only people who have willingly chosen to send a life to Cerridwen are worthy of a second one in her eyes. In this way, common peasants can't be brought back to life themselves.
As for super solders, how powerful they are when they come back could be proportionate to how many people they have killed. This would further encourage generals not to just kill thier soldiers, but wait for them to find a worthy end in battle so that they can bring them back as the strongest version of themselves they might become. Adding the idea of a worthy death would also line up with the other Celtic mythologies so you could introduce things like Valkyries who search the battlefield for fallen heroes to give eternal life to.
In this case, the most common strategy would be to put the mortals in the front-line where they can earn thier divine favor, and to only deploy your undead when needed.
If you do this, you may want to make sure to avoid chain killing exploits where you have each person in a group kill one other so they can all come back. This could be avoided if bringing someone back washes clean the "credit" earned by who ever killed them.