I have a 100% record in ranked solo q with jungle Diana, although it is low elo. Her clear is amazing. Her ganks pre-6 aren't the best, but she has a tonne of presence and if you get that early gank success you'll hit 6 pretty quick.
She has a lot of build flexibility in the jungle as well. If you want more damage you can go Spectral Wraith into Abyssal, Zhonya's, Deathcap etc.
If you want to be more tanky you can either skip the spirit stone upgrade or get ancient golem and go Zhonya's, Abyssal into GA.
A lot of high elo junglers play her in solo queue.
A question I always ask myself when i'm jungling in soloqueue:
"How will my pick benefit the team?"
For Diana:
- Nice and safe clear
- Jungle control through snowballing
- Early-early mid game control through the massive damage spike when you reach 6
And that's it. To me, Diana falls off in the jungle due to farm. She only does well when she takes the kills from the ganks. This goes against my jungle rationale. I always give my laners kills because I want them to snowball the lane to secure towers
(check my jungle stats. Most of my junglers are like 3/3/13). It's hard to play a snowball jungler right because you spend a majority of the game in the jungle and waiting for ganks to happen. If you don't use the gold from your snowball properly, you just doomed your team.
She also can't compete with other bruiser/tank-utility junglers during the mid and late game phase in terms of peel. Sure, she has the damage but that's only for early and mid game. There's no way a team will let you reach their carries for free. I guess you can win games by catching people in the jungle but that rarely happens. Teams always group up at the 25~ minute mark.
Diana also sucks at getting dragon. My entire jungle play is based on dragon control. This is why my top laner is usually doing very poorly if s/he gets camped. I almost never help top lane after my second clear. I always stay near dragon after the 10 minute mark with a pink ward.
If the enemy jungler is top, I can always get dragon. This is why I play junglers like Vi, xin, hecarim and Nasus. They can solo dragon. It is very hard for you to mess up jungling if you can solo dragon at crucial times.
And Diana ganks are pretty bad pre-6 compared to the other junglers right now. She can counter gank pretty well though.
Jungle Diana just doesn't fit my play style. And there are other junglers who fit into more team comps.