Why The Celestials Made Eternals With Terrible Powers
If the main purpose of the Eternals was to protect humans from the Deviants, then it's fair to wonder why the Celestials didn't give them more appropriate powers. Eternals came to MCU Phase 4 and provided a massive upgrade in regard to power levels and in-universe history. Whereas alien races like the Asgardians and the Kree seemed to be ages ahead of humans when it came to physical prowess and technology, the Eternals are well above even then. The Eternals are godlike entities who could easily take over the world but follow the order not to intervene in mundane affairs.
In the MCU, modern society owes a major part of its growth to the Eternals, who utilized their abilities to push humanity forward while the Celestial Tiamut gestated in secret. One Eternal that was key in this process was Phastos, whose ability to design high-tech inventions allowed the world to jump ahead in terms of technology and scientific progress. Other Eternals like Ikaris and Gilgamesh were blessed with powers like flight and super strength to keep the Deviants at bay should they ever appear.
However, some Eternals couldn't contribute as much as Phastos, Ikaris, and Gilgamesh. For instance, Sprite's illusions can barely help her distract the Deviants for a short while when fighting alongside Sersi and Ikaris. Similarly, Ajak's healing abilities are helpful for other Eternals, but she can't stand her own against multiple Deviants, and she can't really defend herself from Ikaris' backstabbing, either. Druig's ability to manipulate minds makes him one of the most powerful Eternals, but since he doesn't seem to be able to mind-control the Deviants themselves, the best he can do is move humans away from danger. On top of that, Druig is not allowed to control humans directly, so his mighty powers mostly end up going to waste. It seems odd that so many of their powers granted by the Celestials are unhelpful considering the nature of the Deviant threat.
It's possible that the Celestials were limited by their own power during the creation of the Eternals, or that the Eternals' stable minds required more resources. After all, the Deviants, who were created before the Eternals, lack empathy and self-control but do have the unique ability to absorb and combine superpowers. If the Celestials' power were infinite, they could have made the Eternals invincible or given them all the abilities they could conceive. This is the case in the comics, where every Eternal shares the same powerset as the rest. But considering how long it takes for the Eternals to fulfill their purpose, it's likely that the Celestials couldn't do much more to help them.
The Eternals had to be underpowered for their live-action adaptation. Otherwise, the suspension of disbelief would be hard to maintain. Still, the Eternals' MCU powers do fall a bit short of the scope of their goals. Given how powerful mortal individuals like Captain Marvel, Scarlet Witch, Doctor Strange, and Hulk can become in the MCU, the Celestials' inability to create an invincible species seems underwhelming.