I'm sure you will get many differing opinions, here are some of mine having owned two previous Ford E-series Class C units and moving up to my Seneca in early 2015.
Freightliner M2 chassis - very robust
Very good "road manners", I upgraded my front shocks to the current Seneca J-Ride units and it made it even better.
Cummins 6.7L ISB diesel - powerful (enough), proven reliable, good economy for power rating
Allison 6-speed transmission - proven reliable
This engine/chassis combination are easy to service with tip-up front clip, I can change my oil, oil filter, and fuel filters in my driveway in about an hour. Service available at hundreds of Freightliner or other heavy truck facilities. When the service people see it is an M2 chassis they smile compared to other types of chassis. Not every Ford dealer is equipped (or wants) to service Class A units.
2 cab doors in addition to the coach entry door
Class C sleeping capacity - I can sleep 6 adults (3 couples) in comfort, plus additional children using the dinette and fold-down couch.
Tremendous capacity to carry "stuff" - My R.O.C.C. is over 5,000 lbs.
Towing capacity - 10,000 lb. rated hitch, actual "safe" capacity is dependent on how the coach is loaded but at least 5K if coach is loaded to GVWR.
Cab protection in case of crash - Lots of aluminum, steel, and cast iron up in front of me!
I like driving a truck, not a living room. My very subjective opinion here!
Great view of the road.
Four coach batteries standard
Ability to fuel at truck diesel lanes instead of maneuvering around tight gas pumps
Higher initial cost
Service consumables such as oil and filters greater (but intervals are generally longer).
Unit very tall (my unit is 13'1" per Jayco)
Cab area not part of living space
Necessary to fuel tanks on both sides of cab
Accessible truck diesel lanes are not as common as gas stations
I don't have too many drawbacks, I really love my unit! Certainly more things I like, but this is what I could come up with quickly.
2014 Seneca 37TS
Toads: 2014 Ford Explorer Sport or 2013 Ford Focus ST