I could never make sense of French phonology. I can remember the rules, but none of it seems to make any sense. Why does "beau" sound like "bow"? If I say "e-a-u" really fast, it doesn't naturally produce an "oh" sound. That's supposed to be the purpose of diph-/triphthongs - to produce a natural blending of sounds.
In Japanese, if you say "nai," it sounds like "nigh." "A" and "I" are pronounced "ah" and "ee," respectively. When you put the sounds together and say them quickly, they produce the "eye" sound. That makes sense. The person speaking doesn't have to remember some specific, phonological rule - the sound is produced naturally. With French, it's like you have to remember phonological rules for about ~40 diphthongs, triphthongs and digraphs. ;_;