Why Is The Ten Tails Beast so Powerful?
The Ten-Tails beast in Naruto also known as Juubi is the Divine Tree that got on a rampage after Otsutsuki Kaguya ate the Chakra Fruit from the tree. Also, this ten tail form is the previous form of all the Nine tailed breasts along with the Redo Statue combined.
Redo Statue is a tailed beast and it has a lot of chakra reserved that Otsutsuki Hagoromo had to divide it into nine equal parts. The Ten-Tails was truly a monster on a whole new level and league.
However, apart from the ten tail large chakra reserve it possessed that made Kurama, Gyuki and Shukaku look weak and tiny. The Ten-Tails also possessed the Rinnesharingan as well.
This looks like a cross between the Rinnegan and Sharingan however, had more tomoe and was bright red visible known as a Kekkei Moura. The Kekkei Moura is a very powerful Jutsu that dwarfed every other Jutsu throughout the entire ninja world in Naruto even the Three Great Dojustu.
Ten Tails Chakra Reserves
Nonetheless, again the Ten-Tails had super large chakra reserves, enormous in size, a very powerful Dojutsu. This can make a Tailed Beast Bomb more powerful than any tail beast bomb combined. Also, is far bigger than the Tailed Beast Bomb of the Nine tailed beasts Kurama.
For example, we saw how the ten tail beast bomb wiped out more than half of the entire Shinobi Alliance in the previous episodes and was able to cast the Infinite Tsukuyomi. As we know, the Infinite Tsukuyomi is a very powerful Genjustu that is able to put the entire world in a Genjutsu dream, making it a legend and one of the most Overpowered (OP) techniques in the shinobi world.

Why Is The Ten Tails Beast so Powerful?
Reseason the ten tail beast is so powerful is cause its destructive power, speed, strength and durability are absolutely insane compared to other tail beasts.
Not even Naruto or Sasuke could beat the ten tail on their own. Even with their current reservoir of powers.
However, the complete form of Ten Tails is a far different stronger and for a different story. Back then it was so powerful that it went head to head with Hagoromo and his slightly weaker brother Hamura which give them an extremely hard battle and defeat.