To me i think there's a simple explanation, i think the Haki Barrier is not very good against straight up physical attacks that are empowered by Haki, such as Thunder Bagua, but it's best used to block long range attacks created by elements or shockwaves, stuff like that, that aren't very solid and only cause real damage by touching the target, so the Haki barrier just repels it, but a physical attack would break it fairly easy.