I tend to agree with Darth about Gurren's crystals. It took Naruto's new combo jutsu to break them when Gurren was actively maintaining that giant barrier thing, and from what she said to Yuukimaru it seems the strength of the crystals is connected to her in some way. I would hypothesize that she has to subconsciously feed chakra to the crystals to maintain them at above natural hardness levels.

As to their strength when she is trying I can't really justify using a loss to a bijuu as an indicator that her technique is weak. That's like saying a top ranked professional sports team is crappy at their sport because they lost to a team of genetically enhanced cyborgs fueled by magic powers and alien technology.