More cylinders means more pollution and more pumping losses and so poorer fuel economy for two similar displacement gas engines. There is a website carcomplaints.com that I have found very useful in the past. It provides information by model year and engine type so one can see if a particular engine had reliability issues and whether this was fixed by the manufacturer.
With my own Chevy SUV and its 3.6L V-6 engine there was a design flaw that resulted in inadequate lubrication of the timing chain and very expensive repairs. This affected this engine from its inception until 2012. Chevy supposedly fixed the problem in 2010 but this is not evident in the numbers of customers reporting repair issues with the engine.
Overall, gas engine reliability increased tremendously when the oil refineries stopped putting tetraethyl lead in gasoline to reduce pre-ignition problems. Now it is a matter of air pumps and fuel injectors and alternators and AC and seals failing or electronics and transmission problems.