[How risky is it using RO sprites in another game?]

it's just the same as "Private Servers" in ragnarok ,

gravity tends to hunt them down backthen , but nowadays private server seems just fine everywhere ,

but they're still in they grey area .

the safest route of not getting tackled is  , taking "donations" by paypal etc , by law you can't be sued by "profittable law" even if u copied other's content if u just use "donations" system instead of "item mall" or in game money currency ,

which im pretty sure , that is what these two pokemon games u said are doing right now ,

with this , you guys are still in the grey area . but safer 

[How risky is it using RO sprites in another game?]

i've been developing some games myself ,

there's no way any developer / content maker can sue you for anything that's non profit ,

logical comparison is like : if you ain't putting any ads in your youtube videos , you can freely use anyone's video as long as your videos ain't against youtube rules .

but if by any chance they know that you're making profit with it , then yes , they can sue you easily