I've tried it multiple times with all manner of builds from semi-tanky such as Runic-IBG-Abyssal etc. to full AP like Runic-Nashor-LB, Runic-LB-Protobelt etc. I think overall, he's not far from being decent but still subpar. He's so bad early 1v1 that I even died to an Eve on equal items and gold ~ level 4.
1. while there are many clear tricks, he still takes a lot of damage early and has mana issues - even 3-5 armor could help him for the JG role... he gets beat up while almost all the meta junglers can clear at 80% HP. Something else that would help is also just buffing his innate AS 5%.
2. He is squishy as hell building Runic Echoes and has no defense or CDR. If he used a few spells on a camp and he's not 6, he's likely just dead to a Rengar, Lee, Shaco etc. You can't count on W due tot he time it takes.
3. Awkward itemization for CDR and mana. If you go LB 2nd to help with the mana issues, the squishiness compared to Protobelt is VERY punishing vs some team comps (such as with Zed, Talon etc.). If you go Protobelt, you basically have to take Blues from your mid. If you go IBG, it's fine but not really so suitable for a heavy AP build and it falls off hard in damage. You may ask, what about Rod of Ages or even Abyssal? Rod of Ages takes a long time to stack since you're already getting it delayed after Runic and on JG income. Plus, it doesn't have CDR. It's hard fitting in all the stats while going JG and also having the big ticket items. Liandry's back when it had 15 Mpen was really nice for a lot of the hybrid builds because it let you get just enough damage to be relevant. Now it's hard and the fact that Runic Echoes lacks CDR makes it annoying.
Sorc (but the JG really beats him up compared to Ninja)
LB (good if snowballing and fixes mana issues + CDR but again a bit awkward if even or slightly behind since you get nearly 1-shout without ult. even by an AP Gragas)
This build is fine but only 20% CDR and still squishy as hell. The HP is pretty meaningless with no resists. I'd really like an item sort of like Rod of Ages but with 20% CDR for AP mages.
Anyway, the bigger issue with Ekko is just that his early is awful and while his mid game is quite good, he falls off again late game because of how unreliable and squishy he is.
I realize I also suck at him but if I play a higher tier jungler of a similar style, like Evelynn, my performance is much better. CDR also doesn't matter on her much outside of her ultimate so I can adapt my builds more. For example, I might go LB one game and Liandry's another. I'm not so dependent on itemizing CDR & Mana as a core stat like on Ekko JG.
My advice: 5% more Innate AS + 5 base armor. I don't think this would affect mid so much but it would matter for JG. His JG's 47% winrate in Diamond+ so clearly not that good.