I recently learned the word 運輸{うんゆ}, which has ん immediately followed by ゆ. However, seeing this made me wonder in what way this is different to にゅ. I can't imagine that there's much of a difference in pronunciation because both have n immediately followed by y, so I thought this might be similar to how double consonants use ん instead of っ.
Is there a difference between んや、んよ、んゆ and にゃ、にょ、にゅ, or is it simply a difference in how they're written?