What do you mean by, no ‘polygon limitation’ un unity ? 
FAIK, current gen engines are capable of supporting such things given LOD systems in place, which has been enhanced in UE4 ( from what I see in features ).
I imagine the key Is memory use, so depending on target audience that could be risky ?
No expert here on such matters, just curious, but I’ve seen unity forum posts ( research only because I was curious, NOT because of engine wars!) where people pushed ‘limits’ and things went sllllow. I think saying no polygon limitation is a bit unrealistic ( depending on hardware, hence my comment)
thx
nl