People are reporting 10-15% performance gains using 4.4.1 w/Short GI enabled on ePMP 2.4. I'd say it's very stable if you're using all Cambium radios. There are issues if you're using elevated radios... but these are typically when moving from earlier 4.X builds to 4.4.1. ALSO, some elevated radios don't support short GI.
At the moment 100% of our ePMP network is running 4.4.1 and we have about a dozen e1k 2.4GHz AP's using it and it's working very well.