there was some teething issues when BFS-LGW moved over to a 737, delays ranged from 20-30 minutes for around a week which was caused by issues at LGW... ranging from slow offloading/loading (anyone whos been in a hold of an 319 for too long will struggle with a 737 offload), and like you mentioned EI-BUD, steps at the wrong height, which can cause a delay of a few minutes if they are having trouble getting the right height/angle
however i know BFS crews got caught out as well as they expected the LGW flights to be A319s, but fortunately it only takes a few seconds to drop steps down
dont know what EZY's plans are for their first 319, but due to the fuel efficiency of their aircraft compaired to the 737.. they might be tempted to put it on destinations outside of the UK