I am extremely underwhelmed by the planet cracker. The final explosion is cool, when you blow up a planet, but the journey to get there is such a frigging pain that I'm not particularly sure why you'd use this other than the cool explosion. Area denial?
There's a few issues:
1. It takes a shittonne to even get to the point to research it. We're talking it requires its own unity ascension slot alongside endgame tech alongside more tech to research after both of those.
2. The actual ship is super weak and super slow. And can't fight.
3. It takes in-game YEARS to charge up. Then after charging up at a Snorlax's pace, it then again takes eons to actually fire. Yes, this stupid weapon has two charge up times, both taking way too long.
In the same time just for the thing to fire, you can hire 30 armies and invade the planet.
So honestly I can only see the use of this being to deny a planet to everyone. Not particularly sure what circumstances would need this as you could easily use such a spot for yourself and it's not like at this stage of the game you even care about tech or unity maluses. Maybe if the enemy planet had like.... 20 fortresses on it with 60 garrison armies?
I dunno about the other doomsday weapons. Maybe they're more useful. Unlocking the ascension perk does give you nice CBs against everyone, but also gives everyone CBs against you.