These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Thunder Force
Adaptation Displacement: Remember Thunder Force II on the Genesis / Mega Drive? Yeah, well, remember the Sharp X68000 version that it's a port of?
Badass Decay : The ORN Faust in IV was impervious to all of Rynex's weaponries, even possessing it in the end; in VI it's vulnerable even to Rynex's supposedly-less-powerful mass-produced incarnation, and multiple of them wind up permanently destroyed by Earth and Galaxy Federation forces.
Game Breaker: Free Range in V. Even the manual of the North American version describes it as the strongest weapon in the game and encourages you to use it.
In VI, the only ship you can initially play as starts off with all of its weapons and never loses them, which by Thunder Force standards is pretty broken. In other words, the default ship is more broken than the unlockable Rynex-R.
Thunder Force V's bad ending: Cenes is subject to repeat what happened to Rynex Vasteel, only instead of ejecting from the ship, she is trapped in it.
The good ending as well. While heartwarming music plays, the Last Letter scene is still a bit spooky.
Enemy/boss design in the games are often pretty Nightmare Fuel-y too, like the Evil Core in IV's eighth stage—a putrescent-looking thing that's served by little white flies that explode. And in its final phase (where a single giant fly is carrying it around), it basically drools its attacks out.
Porting Disaster: For some reason, Thunder Force AC has a really slow autofire rate, forcing players to furiously mash the fire button to achieve the same rate of fire as the Genesis version's autofire. And then there's Thunder Spirits, a port of AC with still no autofire and lots of slowdown.
Thunder Spirits actually has autofire, it just has to be turned on in the options menu, which for some reason is hidden instead of being obviously accessible.
Ostensibly this was all to make the game more challenging. It does, but probably not in the way that was intended.
Sequel Displacement: Few gamers have heard of the original Thunder Force (it doesn't help that it was released in 1983 on an obscure Japan-only computer platform), and even fewer have played it.
Sequelitis: VI was released 10 years after V, and came to be a big disappointment among fans, with only six short stages, a radically different soundtrack, excessive homages to past games, weapons as broken as Thunder Force V's, and a new weapon mechanic in which you start with every weapon and never lose any of them.