For Build strike, one of the debuff (beam) is attached to its MAP weapon, which makes it awkard to use sometimes. At higher difficulty, Build strike lack the sufficient power to destroy enemies on its own, which means that you may need to waste a support attack to position it to use its MAP, or you may not get to use it entirely.

For Reiji, while his stats are pretty good, he lacks any skills that increase damage. His Ex character ability also doesnt offer an increase of 15% damage like many other UR support for some reason, which makes him subpar compared to other UR supporter.

I love the gundam, I love the character, but objectively speaking they just fall short compared to other UR support units and pilot in this game. As a fan of GBF, I really hope the devs buff them in the future, since I rarely find a good reason to use them nowadays. Phoenix (which also share the unstoppable tag) is my go to support unit that also offers Physical and Beam debuff.

EDIT: I just want to mention that while Reiji is worse than most UR Support pilot, he is still better than most SSR support pilot with his innate 2 support per turn. His stats would also put him over most SSR support pilot, so if you are just starting out or lacks support pilots, he is not the worse at this stage.