Uatu has been a constant in the canon series of What If..? and he has been watching the different alternate realities all this time, knowing what happened and what is supposed to, and he is no simple character as he is powerful as well. As seen in Episode 8, he is strong enough to face one of the hardest-to-beat villains, showing that he is beyond most heroes in the Marvel Universe. Here is an analysis of his strength from the series.
Uatu The Watcher has been there from the beginning, sharing what he witnessed from the various alternate realities, but in Episode 8 of What If..?, we finally see him engage and he showed that he is no ordinary character looking over the multiverse, that he is there for a reason.
When all of life was destroyed, Ultron sits in silence as he emerges victorious in his goal of bringing peace. However, he became aware of the existence of The Watcher and Uatu was surprised when Ultron looked at him and noticed his presence. Ultron found out that other universes exist and planned to find Uatu wherever he hides. He was surprised how Ultron could do that.
Ultron was able to break through his place and they had a battle. Uatu is so strong that he could counter and face Ultron despite him holding six Infinity Stones. Ultron asked why Uatu just kept watching and The Watcher exclaimed that he swore an oath against intervention on the natural order of things. Ultron claimed to be the natural order himself, bringing peace, as such is his purpose.
Ultron mocked Uatu's will for not being enough and from then on, Uatu's switch was flipped and he transformed, like going he's going to be a Super Saiyan in Dragon Ball, Uatu changed and exposed how powerful he really is in a gold and purple armor.
Uatu shows how strong he is, he faced Ultron and made a shield for himself. The evil AI had six Infinity stones in his possession and yet, Uatu levels his strength, he is at par with the six stones altogether, which only means he is beyond any character that we have already seen in the MCU. He could jump from one reality to another and could control the multiverse.
He is an extremely powerful being in the MCU, but his biggest strength and weakness at the same time is his will. It all boils down to this. He watched through various realities and despite wanting to intervene, his will to hold on to his oath is too strong. At the same time, it is his greatest weakness, that even when the danger is on the multiverse already, he still holds on to non-intervention.
All episodes of Marvel's What If...? Season 1 are available to stream on Disney+.