Remember that there's several ways to take out a buffed unit from the infamous Killing Machine's soul skill.
Penetrate, and probably a lot of other skills that ignore the defense, such as those spear units from Refess starter deck. You have this fire storm likish grimoire unit that do multiple attacks on random units, which may work wonders if the opponent only have one, or two units on the field.
Although, I do agree that of what I have seen of the starter decks, the Killing Machine is a lot stronger than most cards in all of the starter decks. I have no idea of how the cards will be later, but time will tell.
However, Killing Machine should really have a limit to how high defense that you can gain from the Cemetary. The unrealistic max would be 120 defense (24 * 5). That is, if anyone would go with 25 units and spare one for the buffing. I'd say that the max should never go over a limit, which should be kept low, since if you put the SS on a defensive unit, it would be its defense + the buff. Which then would result in a tank. But still it would be taken down with flesh recycle, and various ignore-defense-actions.
Also the best counter towards the LeBeaus SS may be that Stalking Death SS on some undead ninja card. "Send one of your units to the graveyard, and send one of the opponent's units to the graveyard".
Since it's mostly used by many higher ups, it sounds like it may be one of the most powerful buffs for someone's defense.
Anyways, I've been able to handle many of these buffs by just using ignore-defense-actions. Such as some unit of Lycantrophe, deals around 80 damage while ignoring defense, while I have others that shortens the life, and counter attack in the most desperate way to deal with those. So, they are far from invincible, but certainly are cheap ways to counter many situations. The team will certainly balance this out, if they don't that may mean there are a lot of cards that we don't know about, or expansions that will deal with this problem.
It may be a popular card due to the fact that it's mainly in one of the starter decks, as well.
Edit: One thing that makes this also too powerful, is if you precisely time the summoning of an Elite Crescent Knight, with Lebeaus SS, you will get a mighty unit with buff_deff + unit_deff, since it's level four, also means Return won't work. Dragon's Tactics can also be countered for a round with that Disengage card from the Lawtia starter deck. If someone would have a card that counters grimoires, and be able to predict (it isn't that hard in several obvious situations) when the opponent will cast out an grimoire like tempest or that fire storm likish spell.
How I would time: The best way to time the Crescent Knight with the SS correctly, is that you let a unit fall on purpose, that draws away 1 LP, enough to activate your defense buff on the unit that you just summoned (in the same turn as you activate the SS). It's very easy, and very devastating.
Also, you want as many units in the cemetary, but not those who may be of use in order to kill. So basically, do not let the enemy touch the LP, and let your LP be decreased until 1 LP away from LeBeau's SS, then at that round, you pick Elite Crescent Knight (since those who fell in the round before gave you enough SP), and then you simply let your only/ or one of the units die. Then at the next round:
You simply put out the Elite Crescent Knight.
Your creature, dies.
You lose 1 LP
You can now activate the LeBeau SS on Elite Crescent without any enemy being able to attack it before.
Simply powerful, if you get a defense with +45. It will result in +55 Defense, 50 HP and 30 Attack, and some heavy abilities.
Predict strong grimoire spells (Usually lvl 4): Your Crescent Knight has 30 HP left, your opponent has saved up 4 sp, and has 4 levels into its tree. I'd take the chance that this user might be planning to choose a...tempest, put your "counter grimoire card"ish on.
If you don't have something like that, take notice if the opponent saves up SP, if you are hurt, good time to Rest, even if it means losing an opportunity to hit.
If you had these defenses, then it would be very hard for any opponent to counter your buff, at least from my newbie perspectives. Just one LeBeau can turn the tide around, if well timed. I've noticed several players not being able to play LeBeau correctly. Where others need 3 in Soul Slots, while you can depend on a good planned one.
That's just my newbie opinion, the SS should have a limit to how high the defense can get, I'd say 20-25 since it's stackable, a damage reduction with 20-25 is quite high actually, and if stacked even higher. But that wouldn't be the best idea, since you will lose one extra addition for a soul skill in order to be more versatile.
Sorry for making a long post.