Unreal 4 is a work in progress so until it’s stable one should consider their projects as first adopters tied to the current upgrade and Epics road map. I can at least verify that the overall lighting and materials have been improving with each release, more so in 4.8 preview, and the rumor is the large world kite map will be released with the gold version of 4.8 so as to the current state of the art it will make it easier for those with large scale projects in mind make an informed decision.
Opinion wise if a project starting now has a 3-4 year development cycle I see no issues as to scaling the project as needed as features are made available and and rather pleased with the foliage improvements from 4.7 to 4.8. Granted Ue4 is not what I would consider a fit to finish engine but if you install future proofing into you design objective then it can grow as the engine features grow and considering the front costs is a more than fair trade.