Unfallen have access to their base Medium Hulls as shown by their manpower(300/300). If they were enhanced or had Manpower modules installed, that number would be 450(enhanced) or larger. They have Beam Weaponry installed on their ships, making them offensively viable at any range, with their defenses tailored towards Kinetic Weapons(meaning they're completely helpless against Energy weapons).

You have three ways to handle this situation:

1) Build more smaller ship hulls and overwhelm the enemy with sheer numbers, exploiting their weakness to Energy based weapons(Lasers/Beams).

2) Build more smaller ship hulls and utilize Vaulter's unique perk with Boarding Pods(Energy Based ships are completely helpless to Boarding Pods). Utilizing this strategy will allow you to capture their ships and use them against them but you will be unable to retrofit the ships with better Modules should you be successful in capturing them.

3) Unlock and build Medium class ships yourself and counter their firepower with your own. 800 Offensive firepower on a Hunter class ship is frankly weak... even if its Beam Weaponry.

It is not a bug and it appears they went towards unlocking Ship Hulls as soon as possible. To make your ships as strong as theirs, see #3 and to defeat it, see #1 and #2. AI is typically aggressive on harder difficulties(even for Pacifists like Unfallen) and I recommend utilizing a Hero as a Fleet Admiral to give your ships an extra edge in battle(while also giving your Hero valuable experience).

GTI wrote:

Thank you Barf, I found the hulls ! I disguised my fleet as pirates and I will try to go through the system the Unfallens are blocking. Is it possible to deceive them being disguised? I've been playing in the normal difficult level and its my first campaign.

They may still attack you despite being disguised as a neutral party. I've found that the best counter to enemies blockading a system is the Umbral Choir's Ghost trait or installing Cloaking modules on your ships. If they cant detect you, they can't stop you but it only works if your cloak is one step ahead of their anti-cloaking measures. Furthermore, Cloak modules allow you to engage the enemy at your desired range based on the battle tactic you've chosen. Meaning if you chose Medium range, your fleet will remain cloaked in that particular lane until it reaches Medium range and opens fire. That way it avoids taking damage from the enemy but that only works if the enemy can't detect cloaked ships. The best way to counter cloak is to install anti-cloak modules on your Hero ships(ideally Seeker class) so that the fleet can spot them right away(and the AI is notorious for using Cloaking Modules).

GTI wrote:

Thank you Barf ! the pirate disguise didn't work, but I'm almost finishing researching the hunter hull to send their fleet to oblivion.

Be sure to have a few Protector or Coordinators to soak up enemy fire for your Hunter ships. Happy hunting.