Was FC2 the Dunia engine or the CryEngine? I don't remember...
Dunia.
The Dunia engine has been used in all the FarCry games since #2. It's based on CryEngine but was significantly modified. I don't know if the Dunia engine received many updates since then though (although of course I would assume it has, over the years, especially with changing industry standards), but I can imagine that FC5 has to be running on the 'latest' version of it. Now when it comes to FarCry 2... well, it wasn't very good, but it had some aspects that were well done. I liked its environments (mostly, although at some point it does become visually repetitive), its atmosphere, the graphics in general were pretty good (especially back then, but still hold its ground well to this day in my opinion) and indeed the physics were well done.
However, the gameplay quickly became a chore for me, with respawning bandits in all camps after a (way too) short period, constant coming back road patrols, generally-idiotic A.I. (which of course persisted throughout the entire series, FC5 being no exception whatsoever), the heavy reliance of vehicles for quick traveling, and the absolute worst offender being the malaria 'attacks' which was unberable to endure after the first couple of times; which they could have simply just shown to us in cut scenes to make the player understand that the protagonist may have his days counted and needs to act "quickly", figuratively speaking for the sake of story-telling. Instead, they thought that implementing the disease into actual 'sudden attacks' that blurs vision, require action to temporarily medicate AND can actually kill you if you don't do the steps to get rid of it momentarily
was a good idea. To me, it made zero sense in an actual gameplay "mechanic". That would be like playing a typical 2D Mario Brothers platformer with Mario suddenly sometimes having paralysis attacks, forcing you to stop, consume some items, wait until the screen gets back to normal and Mario can jump again to proceed, otherwise you die right there on the spot. It was completely unnecessary and could have stayed a story-based, scripted thing that happened only during crucial character progression-related moments on a few occasions during the main missions.
Anyway, for me FarCry stopped being interesting at #3 (which I simply couldn't finish, I stopped at the point where you meet the exotic girl for the first time, right after that I stopped; never played it again and I don't miss it one bit), and I actually preferred the spin-off Blood Dragon (which has a fantastic Terminator meets Tron 1980s sci-fi styled Soundtrack).