Jayce is a utility and poke-based champion. His melee abilities and base stats tend to be insufficient for extended use against dedicated melee champions, and not having range against actually ranged champions is of course rather unpreferable. He has some difficulty in his use since his mana-costs are too high to use completely nilly-willy (also, 6 potential abilities to drain your mana with) and being forced to go into melee form to replenish his mana like I mentioned is a bad thing. His main weakness is burst mechanics that will damage him before he can keep away from it. As mentioned, he's got utility. A lot of utility. If the team speed-buff, slowing gap-closer and large knockback didn't peel your squishy, the squishy was either insta-burst down or for some reason unable to right-click away from enemies which is totally not your fault. His ranged Q is also one of the best pokes in the game, and one enhanced by his ranged E is gonna hurt something it hits.
Riven is a quick-moving and heavy-hitting snowballer. Riven is usually able to get up to stuff she wants to get up to, get away from things she doesn't want to get up to, and her attack damage-scaling makes her smash faces when she has any advantage against most opponents. She has some utility with a stun and knockback, but they tend to be a bit minor in practice due to short-range and usually just useful for an autoattack or disengage opportunity in a laning duel. Her base stats are very low for a fighter champion and she lacks significant base durability, making her dependent on said snowballing... but she does snowballing spectacularly. The minor utility I mentioned becomes much less so when she has enough attack damage to improve her shield to let her dive into teams and hit them all with that utility while damaging them nicely. Her major weakness is late game teamfights, when most melees won't be killed by her as fast (though she'd still probably gonna win a 1v1... but those ain't happening anymore), ranged carries obliterate anything with a bit of time and some good hard CC will probably have her dead before she can use all of her dashes and using all of her dashes to get in range, when teams tend to see and stay away from each other before they actually start fighting, causes her to lose a lot of damage potential anyway.