If there's one thing I hate as much as side characters that don't amount to much, powers that initially kick ass but don't appear later on since the supposedly more powerful forms take precedence.

I like Pine Arms, but after we get Jimber Lemon it's practically useless.

My biggest beef though goes to OOO Gatakiriba. You can create like 8 clones of yourself and use every single colored Combo form! He uses it in the OOO Movie and it takes out the big bad monster. This is never brought up again! What?! Why? That literally puts Eiji in contention for one of the strongest Kamen Riders in existence!
Ugh...

Something you have to keep in mind with the movies is that they tend to pull stuff straight out their asses.

Blade Jack was used often enough, though him getting King 8 episodes later is why it felt so underused.
Garren used his Jack form plenty, though.

The non Combo forms in OOO is kind of an unfair point, though, since a big factor was the superiority of combos vs the random hodgepodge forms. TaToBa was more balanced than most other forms, so it made sense to not throw in random medals like Cheetah or Gorilla unless necessary.

Because of frickin Grateful Damashii, we don't see the 15 Eyecons anymore. Or even Ore Damashii.

The main reason I believe was CGI budget so they couldn't show Gatakiriba a lot because of it. I believe one of the OOO Net Films/Episodes references this when OOO breaks the 4th wall and addresses why Gatakiriba is called 'The Strongest Combo' but Eiji hardly uses it...

No, he's referring to a scene in one of the movies where OOO creates 8 GataKiriBa clones, then each of them transforms into one of his other Combos, and why he's never done it again (Making the CG problem nil since they would only need to use the CG briefly before swapping over to the main costumes).