Rattleballs is essentially a versatile bruiser that has the ability to turn around teamfights with his high damage output and large-AOE crowd control. He also can be build as a wizard which makes him less effective in lane but even stronger in teamfights. But what makes him so strong in this current patch?
Let's look at the raw stats first. Upon release Rattleballs wasn't too popular because his lower damage than other bruisers (and the fact that he never appeared in the show itself -_-). But in Patch 7/1, he received significant buffs (http://i.imgur.com/gzo5ffq.png) that increased his damage, attack speed, and the effectiveness of his ult. He now has the highest BASE attack speed at level one, and his AD matches those of other bruisers now (50 AD). This adds on to his already strong tank stats: 500 HP, 25 Armor, 20 Shields at level 1.
Next, let's look at his main backpacks, because raw stats don't matter if you don't build him right. Most bruiser backpacks suit him right, offering him some damage along with different tank options. Jake's Mace gives him 50 AD and 50 AB (100 damage, equivalent to Finn's Golden Sword) at 4 points ALONG with 100 bonus health, which is highly underestimated on an AD item. Compare this to bonus health on Robo-Suit, also 100 HP. This makes him pour out damage like most other bruisers, but he's tankier than the others, with higher health per level . People also build him with NUWW, which at Level 10 (4/4/0/0/2) gives him 100 AD + 50 AD, 35 PD + 220 PD (+50 Shield Breach), and 900 HP + 75 HP. He isn't nearly as tanky with this build, but if you apply that PD (305 damage total) to his ult (75 + 0.8 PD), that's a whopping 319 damage PER SECOND for 3.5 secs that can hit multipale targets. If you can hit all three people for the whole 3.5 secs, that's 3349.5 damage you just dealt out (disregarding opponent's shields, but they usually won't build too much shields).
Now, let's look at Rattleballs's abilities. His passive gives him 50% life steal for 2 attacks after using an ability. Early game that's not much but in the late game, where let's say you deal about 200 damage, you heal 100 HP per attack for 2 attacks. His Q is his Shadowless Thrust, which can be used in three ways: Dashing twice, countering an attack, or dealing AOE damage. This makes it useful in many situations such as getting away, countering an attack in a duel (which usually helps you win the duel), or clearing waves and jungle camps. His W is his Dust Storm, which isn't useful damage wise but is your main initiation ability. It's extremely large cast range lets you hit multiple targets, and if you use it corrrectly, you can cancel things like Gunter ult (most common), Beemo stun (occasional), and even LSP ult (sometimes). This also sets up for your teammates to cast their skillshots and for you to activate your ult. Rattleballs's ult is Sword Stuff which is a menace in the early game altar rush and is your main damage source if you build him PD. Easy to set up with your W and useful in chasing because of it's movement speed buff. Again, if you build him PD, it will scale into the late game, but won't scale as hard if you build him AD.
Finally, let's look at how he fits into the meta itself. Most teams have a high-damage carry, such as Gunter and FP. Rattleballs is able to effective assassinate them with his dash, dust storm, and basic attacks. He can even cancel Gunter's ult. Rattleballs AD is just like other bruisers, 50 AD + 4 per level. He doesn't have an attack speed buff like Finn, Fionna, and PepBut, but makes up for it with his high base AS, at level 1, 1300 (Finn, Fionna, PepBut: 1500). But the thing that sets him apart from other bruisers is his tankiness. He has more armor than others, 25 (Others: 20/10), and scales well off of HP due to his passive, which gives him more dueling potential and teamfight durability. One final particular meta-related strength that he has are his counters. In my opinion, Rattleballs's main counters are The Lich, Ice King, and PepBut. Lich and Ice King have strong area control that forces Rattleballs to dash away from (but your dash is usually down after initiation) and you're vulnerable to take a hit if their skillshots land. PepBut is one of the few bruiser counters Rattleballs has because aside from being slightly squishier, PepBut can effectively duel with his blind, stun, and chase down with his ult. In average/high elo, Ice King and Lich get outshined by wizards who offer more damage (Gunter and FP again) and PepBut isn't a popular pick because he's extremely vulnerable to getting kited/poked.
These, in my opinion are reasons of why Rattleballs is strong right now. If CN were to nerf him, I'd suggest reducing his PD ratios and/or nerfing his health/armor/shields to give him some defensive weaknesses. Thanks for reading my analysis on Rattleballs (I know it wasn't a short one), and I hope this helped you use or play against him effectively. :)