Found the answer to my question and I hope it could help out someone who suffers from the same issue. Turns out video resolution is the factor between the simulator and the actual phone when it comes to store video to the gallery; to be specific:
Compatibility checking UIVideoAtPathIsCompatibleWithSavedPhotosAlbum would FAIL on the actual phone but would PASS on the simulator given a sample clip capturrd as 1080p. I came cross this link a week after my post with luck.
"What video formats are compatible with the assets library?"
And I did the experiment similar to that by capturing 720p video clips instead and all files were able to be saved to the gallery. Weird thing is that the actual iphone can play the HLS stream at 1080p; howeve it won't store clips unless they are 720p, not to me anyway. And the misleading bit is that the simulator can actually do 1080p.