Let's get the bad out of the way first.
CONS: Each episode intro shows that the first wide-scale alien attack from "the Neighborhood" dimension was quelled 4-1/2 years ago, but we are never shown exactly how, or how they kept casualties to fewer than 1% of the city's population, or who most of the pioneer heroes who formed the Border Defense Agency were.
Incredibly, most of the current ace fighters are 21 or younger. Kuga, the Neighbor who fled and journeyed to Medeen (Earth) was already his nation's top warrior as a pre-teen. There's a very large ensemble of heroes who shape the story. Tip: use the color-coded team uniforms to keep track of who's who.
The cartoonish pouty faces common to anime are present, but not in any fight scene. There's the obligatory resort spa scene, but 1 fan service in 73 episodes ain't bad. The Xeno/Lilith arc is where the series "jumped the shark" by being too far-fetched and detracts from the show's overall legacy. Thankfully it rebounded.
Now for the great things.
This is an intellectual anime, and unlike other thought-provoking series, you will never get lost if you miss a dialogue; everything is re-explained in the pre-credits. The main character grows from trainee to hero to leader; it's slow and steady, but hey, he's 15. Kuga's development stopped halfway through as he faced a tragedy; still, 37 episodes feels adequate enough for an alien to get used to Japanese life.
Even minus an origin story, there is way more than enough geek science to gratify and at times overwhelm fans. The combat concepts are interesting: transformable weapons (triggers) enhanced by a person's Trion (ala Star Wars' The Force) + body swap into a Trion construct. An agent can de-transform, or if the construct is damaged enough, can "bail out" and rematerialize to their physical bodies back at BODA HQ. A weird ability among trigger wielders who die is become a Black Trigger. As a bonus, a select few have superhuman abilities. I never get tired of Jin's pre-cognition declarations to skeptics: "You will..., my side effect tells me so."
The battles are visually stunning and exceptionally choreographed, even if you fast-forward the commentaries. Villains are underdeveloped but not repetitive; the progression of enemies is satisfying as battle droids become stronger, and their masters, the human-like Neighbors are menacing and increasingly powerful with unique abilities (one took 10 top agents just to force a retreat).
All these qualities converge into non-stop action achieved through the mythos. There are alien skirmishes and 3 additional major invasions. In between, BODA agents prepare and demonstrate improvement through solo and team Rank Wars. You will want to rewatch the Finally and probably the best part, endless cliffhangers! Episode after episode stopped exactly where it should've and kept you wishing for more. Really well-thought out. 10/10