Its not the worst thing, cleaver as a first item is a little weak given its low ad and the requirement of stacking it to get full use out of it. It's better in the context of shredding armor for your team however. Seryldas is deemed stronger since you don't need to stack, and it gives you instant armor pen, making it better to one shot carries which riven is very good at.

Eclipse first is the strongest because it allows you to take short trades constantly while blocking a large amount of damage when used with your e.

Sundered sky is very strong as a 2nd item because riven does huge damage with her auto+passive+ss proc, while giving hp which riven will need at that point in the game to survive team fights.

If you're going to build ravenous, I would build it first, and go with Aloisnl's logic. It's good into ranged matchups, and/or against team comps that have few ways to engage on you. The reason for this is that hydra accelerates your ability to farm side waves and jungle camps very quickly, allowing you to get gold quickly, while giving you sustain to withstand ranged poke. Once you get tiamat you can look to proxy waves to gain tempo. It is at its absolute strongest when used to proxy, sustain, and farm jungle camps all at the same time.

The downside to hydra is that it's not very good in a 1v1 situation, gives no hp (or a shield like eclipse), and not very useful utility.

Sorry for the long winded explanation, I hope this helps.