Any champion can mitigate their weaknesses by building items, so it's fair :V
And if you want to get technical about it, Riven's shield helps her a little bit in the sustain department.
Lifesteal does help, but early game it doesn't help too much if you aren't constantly attacking and have low AD. Health pots are useful, but only give you 10 regen a second, which is useful but it cannot stack with itself and takes time to fill your healthbar. Over reliance on health pots can lead to lower gold income if you're not ahead (not a huge deal but if your opponent has a gold advantage and you're going hard on the pot, you might run into a problem).
So yeah, items help but they don't always 100% solve the problem, especially if you have a similar top laner that builds the same things. If a Riven builds vamp sceptor as her first advanced item, and let's say a Gnar does the same, Gnar has the same level of sustain and more poke, keeping his advantage. If she went for hydra and he went for BORK, he would have an advantage through its active while she would have stronger pushing power.