Good concept. The only flawed part is that it was never said at any point that this was possible (like in every manga) But the concept is really good, creative and easy to understand and with good support. Having a side effect explains well why it was only used now and cannot be used much nor will it be used in the future, and the issue of high resistance is quite obvious since his body is only strong as it is because of the sword, so it makes sense to switch it to give more resistance.