It really depends on the type of weapon the armors are designed to defend against.
Storm Trooper armor from Star Wars seems pretty useless as I have never seen it stop anything, so I have to use general terms.
Most futuristic and modern armor would likely not be designed to stop big melee weapons. They do not need to as such weapons are not used against them. Instead they protect against high speed impacts or energy discharges. In a fantasy environment that would translate into resistance to energy attacks and a big bonus against ranged attacks.
On the other hand, if the futuristic armor is also designed to stop futuristic melee weapons, for example a Warhammer 40K Space Marine or other power armor, this guy would probably be pretty invulnerable to normal, nonmagical attacks as futuristic materials would be harder than any mundane material a low tech fantasy world can produce. Magic would be your only option here as such armor also tends to be sealed so there is no bypassing it or using gas or other indirect attacks.
Generally, carrying comfort would likely be higher in modern armor than fantasy ones (unless the futuristic weapons are so powerful that you need to take extreme measures to defend against them) meaning less skill penalties and maybe faster movement unless it are really big "hulk" like power armors. Speaking of power armors, they usually run on a battery (and also have an air tank if sealed) so unless the PCs have a way to recharge it (and figure it out) it will stop working at some point and thus become so heavy that it is unwearable.
Of course, depending on the weapons the armor is designed to defend against, they can have other interesting properties. For example, armor designed to defeat gyrojet weapons might have a reactive outer layer which will explode whenever a arrow or even melee weapon comes in contact with it, destroying the armor and likely damaging the melee weapon and its wielder. But charges would be limited as once a reactive plate is gone it can't be replaced.