Just spent a month in Vietnam on a heavy engineering project, typically I use 13" or 15" Retina MacBook Pro`s. The Retina MacBook was very much pushed to it`s limits, equally the MacBook got the job done without any drama, running on battery during the day, recharging at night while in use. I never found myself concerned about the Macbook`s ability to get through the day on battery. For some CPU intensive tasks the Core M lagged, equally I deliberately opted for portability over performance for this trip knowing the rMB would be adequate, albeit slower than the 13" & 15" rMBP`s
Positives are clear class leading portability
Downsides are heavily dependant on usage/workflow, outside of the physical smaller display. So in many respects it`s difficult to state with any level of accuracy. For the most part unless you have extreme needs or want to game the Retina MacBook gets the job done. The single port may be an issue to some, for me it presents few if any barriers as I have a USB C, USB A combined flash drive and adaptors. If I required a host of USB peripherals hanging off the notebook I would have used one of my rMBP`s.
Q-6