What I've learned about potting irises versus in the ground: In my locale, irises thrive in pots and do fine over winter. In fact, the problem I have to address is that it's difficult removing the rhizomes and they need thinning by the second year when grown in 14 inch diameter/ 7 gallon pots). My pots are pricey and appear to be of a synthetic material prone to cracking so I have to be extra cautious when digging up and thinning the rhizomes. I put them in pots because there's a wall separating our yard and the neighbor's property and planting in the ground would make it difficult for the plants to get sufficient sun exposure. Thus far, the 'pot experiment' has been successful and a few members this year received rhizomes from those plants and I was thrilled/ relieved that my rhizomes were given good feedback.
