There are thirteen cards in the format with riot. Four commons, five uncommons, one rare, and three mythics.
Most of these cards seem fantastic to me. Even Wrecking Beast, which is a hefty seven mana, can completely turn the game in your favor. The only card that I don't expect to be good is Ghor-Clan Wrecker. Four mana for a 3/3 menace or a 2/2 haste sounds too far below rate, but every other card can really pack some punch. 4/4 haste for five is a card I'd definitely play in most Limited decks. Same with a 5/5 for five. When both riot modes are exciting, I expect the card to be quite good.
Burning-Tree Vandal, Clamor Shaman, and Gruul Beastmaster have a bit more play to them. Attack triggers turn the game between haste or a larger body into an entirely different debate. If you give Burning-Tree Vandal haste but then your opponent casts a 3/3, you're out of luck. You want to be able to attack as many times as possible with these cards in order to maximize their abilities. However, with both Clamor Shaman and Gruul Beastmaster, you may be able to facilitate lethal off just one attack, depending on the gamestate. Regardless, take a minute to think before you play these cards with riot because the wrong decision can change the outcome of the game.
The bread and butter of aggressive decks in Limited is the two-drop slot. You want a minimum of six two-drops, but I'd prefer eight or more to be very happy about my deck. Limited has come a long way, and Grizzly Bears simply doesn't get there anymore (sorry, Feral Maaka). Territorial Boar seems like it has enough upside for a common, and later in the game both Gravel-Hide Goblin and Sauroform Hybrid become real threats that are difficult to block. What makes or breaks an aggressive archetype is the two-drops at common, and I think Gruul is just shy of where I want to be. Admittedly, the uncommons are very good, but it's difficult to tell if that's enough to push the aggressive riot archetype over the top. Gruul may be better suited for a midrange deck that goes a little bigger and plays more cards like Rampaging Rendhorn or Rubblebelt Recluse and hit hard with Sunder Shaman.
After two-drops, the next thing I look at when evaluating an aggressive archetype are the combat tricks. The cheaper the better, but sometimes the cheap tricks don't pack enough punch. Vampire's Zeal was absolutely fantastic in Ixalan, but usually the +2/+2 trick for one mana is only okay. Gruul has access to Storm Strike and Stony Strength. Storm Strike seems lackluster to me, but oh my god can Stony Strength facilitate a blowout. One mana for one +1/+1 counter is fine, but nothing special. However, the untap clause means I can spend one mana, permanently augment my creature, and surprise block my opponent. This card may end up being only okay if that counter doesn't often let you eat a creature in combat, but I have high hopes.
After the cheap tricks, the rest tend to be replaceable, yet fine to include. Gift of Strength isn't great, but if I'm in the market for a trick, I'll play it. Colossus, on the other hand, can kill out of nowhere. Act of Treason and Burn Bright can be good in the right deck but are narrow and are most likely relegated to the sideboard. Overall, the tricks are good but nothing special. I think this still points to Gruul as a more midrange strategy, but given the choice that riot provides, the archetype may just be able to play both roles of an aggressive archetype and a midrange stat-monster archetype.
It's always important to consider the removal that a color combination has available to it. One of the biggest issues with Gruul is the ability to handle larger threats. Savage Smash and Titanic Brawl are the fight spells that can remove big monsters if you have one yourself. It's hard to tell exactly how these removal spells will influence the evaluation of the Gruul archetype, but it's important to note that five toughness is the line where most of these cards will not properly answer the threat.
The last note I want to make about Gruul is that, like Simic, it cares about +1/+1 counters. Riot easily places counters on your creatures, which turns Bolrac-Clan Crusher into a fantastic threat. Trollbred Guardian giving your big guys trample is nothing to scoff at, and alone it's basically a five-mana 7/7 trample. I don't plan on passing that card much. Rumpling Ruin requires a bit more than I would like, but at least it's on a large body! Overall, I expect these cards to be at their best if you can delve into Temur colors thanks to the wonderful fixing in the set, but they all have a reasonable floor.
I'm excited to see where Gruul lands in this set. I have a bias towards aggressive archetypes, so I'm hoping I can curve out with haste creatures and bash face; however, after looking into the cards available, the more I think that Gruul will be best as a normal midrange deck. Below is an example of a good Gruul deck in Ravnica Allegiance Limited. It's aggressively slanted, but nothing like how fast Boros was in Guilds of Ravnica.