To my knowledge there is no advantage to purging with a hydrogen mix. Shielding with hydrogen is a different matter. The disadvantage of purging with hydrogen I think is BOOOOOOM!!!!!
90/10 as shielding is called Formier gas and was once common. The hydrogen allows greater speed of welding and penetration and provides cleaner roots as well. It is exclusively, to my knowledge, used with GTAW (at least in the common fusion processes)and as shielding only for roots.
It has fallen into disfavor I think due to H2 cracking concerns but may still be used from time to time with alloys not as susceptible. Not a recommendation on my part. I've never used it but I can see where it would have distinct advantages.
As for nitrogen, its cheap and can be used as a purge, but it is an active gas and the alloy being welded should be considered.