Eliacube might be able to reverse the time to 200 years ago, but
It has a small energy (Wakfu) efficiency, so a lot of Wakfu is lost when Nox is trying to reverse the time. Nox's technology and Nox himself are much insufficient to utilize Eliacube, so it causes only wastage.
Let's use an example to explain. Think of a circuit with a battery, a electric-conductive wire and a lamp. Now, Eliacube is the battery, Nox himself is the wire, the history reversing is the lamp(going to light up). Here comes a problem: this battery is too strong, yet the wire is pretty much weak. 100% output from the battery(Eliacube), some of the power lost in transferring via the wire (Nox), some of the power lost in reversing the history(to return the Wakfu to the source, mentioned below). In this case, maybe the real power efficiency is lower than 5%.
Nox is reversing the history, so the wakfu taken will also be reversed and go back to the source, and thus it is "depleted".
So Nox is right when he says that all the destruction he has done will be reversed: ironically enough, even the wakfu he has taken also returns to the source as the result of reversing the time flow.
It may not even be a matter of efficiency, but simply the scale of what Nox is trying to accomplish. When Nox actually casts his spell, we see an orb of light expand until it covers entire regions of outer space. Whether he understands the true scale of things or not, Nox is trying to turn back time for the entire Krosmos, with the Wakfu gathered from some two hundred years of murder on one tiny planet. It's like trying to run the entire planet Earth off of your local power plant- or your backyard propane grill. The really scary part is that it works. The Eliacube is actually powerful enough to affect the Krosmos.
Qilby's weapon of choice is a Sinister Scythe, what are scythes used for? Harvesting.
Sadlygrove's death on episode 25 is true and he isn't supposed to come back alive, but then Nox, after his victory against Yugo, reverses the time for 20 minutes: Just enough for Sadlygrove to disappear again, but this time, as shown in Season 2, the history changes a little and thus he ends up getting 'saved' by Rubilax.
Baby!Yugo was using his future trademark hat a baby sleeping bag when Grougaloragran brought him to Alibert. So when Yugo grew too big to sleep inside, he started to use it as a hat. This also explains why Yugo's hat is similar to Eliatropes' traditional hats while Yugo and Alibert had never seen Eliatropes earlier.
In Season 2 episode 5, a starving Yugo, Ruel, and Sadlygrove encounter, and subsequently devour, a small winged pig in the wild. Later in the episode, Yugo, Amalia, and Eva get turned into small, winged pigs by a trap in a dungeon. Makes you wonder where that first pig came from — and the end credits in particular seems eager to remind us this.
Also; in the OAV it appears that the human members of the Council of Six are aging much slowly(Yugo for example). He angst a little bit because he is in love with Amalia and too insecure about his childlike appearance. Now let's think a bit: Qilby is pretty damn old and he can't forget his past lives (even says that he died), does that mean he also remembers all the women he has outlived?
It's revealed that Sadlygrove's emotional state plays a hand in how he acts when fully possessed by Rubilax. That means there was a part of Sadlygrove that was totally fine with trying to kill a twelve-year-old boy who had the guts to stand up to him. Sadlygrove was at a pretty low, lonely moment when he was possessed...
Also, when comparing his Character Development with his master's, you will find very creepy similarities: both start being dumb and incompetent fighters and then gradually became great warriors. They fall in love with a woman, have kids and then finally wed. The big difference is that Goultard has lost his entire family to a possessed lunatic while Sadlygrove hasn't. Yet...