1 :

Earth already experienced a great extinction from an invasive species 2.4 billion years ago that killed 99% of all life on earth. It produces a chemical that was highly toxic to nearly all other lifeforms. That phylum is still the most plentiful lifeform on earth. (That chemical is oxygen and the lifeform is cyanobacteria.)

Today, we are still dealing with lifeforms that are constantly trying to convert the entire biomass of the planet to themselves.

Can you guess what that lifeform is? It's every single living organism, from the smallest bacterium to us humans. Every organism has evolved over billions of years to optimize the conversion of inorganic matter and other living organisms into copies of itself. The introduction of a new, alien species that dramatically outcompetes other lifeforms in every ecosystem on a purely evolutionary basis would be a quite difficult problem.

To take a small flaw as an example: the "White Claw" invaders in the film can take down top predators - and they can also glide. But flight requires light, hollow bones, whereas brute power to take down large animals and throw military vehicles like matchsticks requires massively strong bones and bulky muscles. The laws of physics constrain all life to specific ecosystems because all life faces all sorts of compromises.

2 :

In The Tomorrow War, the invaders compete with humans by a non-intelligent (or at least low-intelligence), non-tool using species. Whatever consciousness they have, the White Claws presumably do not adapt strategy or technology to the human responses.

This summer, I've been trying to get rid of weeds in my backyard. At first, I tried pulling them by hand, but they grow back. Then I tried an edger, but it does not destroy the roots, so they grow back quickly. Then I tried 30% concentrated vinegar, and it works because it's absorbed by the roots and kills the plant. While the weeds might eventually adapt to vinegar, the point is that I can change technology much faster than the weeds can evolve new defenses.

Humans have been dealing with invasive species for as long as agriculture has existed. Our key weapon in the fight is our technology. We determine which strategy works and then scale it up. We don't keep using the same failed methods (small arms fire), like the protagonists of The Tomorrow War.

What should we have done to deal with the White Claws? Well, wasting resources to firebomb them when they already dominate an ecosystem is stupid. Instead, all initial resources should have been spent to identify a viable defense method, then scale it up. Even within the movie, it only took a day to find a toxin -- why did they only think of that in the last gasp of the war? If not a toxin, then what about armored vehicles?

Here's another problematic aspect of big dumb animal invader science fiction: living beings are still made of blood and guts. Physics puts upper limits on density and power so that a tank will always be able to take out a big dumb biological being -- and do it beyond line of sight.

A small, hidden invader is much more difficult to defend against. A virus, bacterium, or even something mosquito-sized is a much scarier threat than a big dumb animal. Mosquitos have been around for over 200 million years, and their victims still haven't been able to mount an effective defense.