Nope you don't lose functionally. EIGRP is coming to FP soon. FlexConfig is
used to accelerate the process of implementing FP. It will take cisco
sometime to migrate ASA features to FP.
FlexConfig alone shouldn't be your reason for migrating from EIGRP to OSPF
unless you have other design reasons. I have implemented it and its working
Same concept if you want to configure DNS servers in FP. At this point of
time you should use FlexConfig to implement. Its not configurable from FMC.
There are many other features.