Hearthpwn

10 damage isn't enough to have a big enough impact on the board. The only way this could work out is if the meta is full of cheap low health aggro minions. The new hunter quest seems to support having a board of dispensable minions, but it also has a ton of tools to refill the board after a clear to progress the quest (the new locust summoning card, unleash the hounds, lynx's, etc) to the point where you're probably better off running more board clears. and assuming your enemy isn't playing aggro, its not uncommon to have 2-3 minions on board with 5+ health, which makes it unlikely that Reno kills anything.

The new Genie highlander card is great for the highlander style of deck, but if it turns out to be too good then people are just going to end up playing bomb warrior to counter it.

I think where this expansion stands out isn't in the legendary cards, but in the commons and rares. a lot of the legionaries have pretty meh effects, whereas some of the new spells and minions are absolutely insane.

I think that the people saying this card are good are comparing this to Volcano, Meteorologist or possibly Dyn-o-matic
and yeah, those cards were good, but they weren't legendary, heck, they weren't even epic.
dropping this on turn 6 or turn 5 might clear the board in some cases, but definitely not all of them, or at the very least when your opponent is far enough on board advantage that you really need a clear, 10 damage probably won't do it.

Worse of all is the deck limitation for this card. Meteorologist does comparable damage for having a decent hand size, and it hits face. Dyn-o-matic only hits non-mechs, but running an all mech deck isn't that bad of a limitation, and it's not like people were building decks around dynomatic because he was THAT good. on top of ALL OF THIS, a bomb warrior has the potential to shut you down in one turn by playing a number of different cards at any point before you play reno:Wrenchcalibur, Clockwork Goblin, Augmented Elekk. Just playing the wrenchcaliber on turn 4 can completely shut down your deck, let alone the other cards in their deck, which they have 2 of (assuming they run all of the bomb cards, almost a third of their deck directly counters yours).

Just a wrap up of why this card is so bad: It's effects are directly comparable to several RARE cards, and it comes with the HEAVY restriction on your deck, with no current support for standard highlander.

EDIT: I just want to add on a comparison to the other new cards that mage got.
Reno the Relicologist fills a role of board clear, but the new cards we got look a bit better.

Compare it to the epic card Tortollan Pilgrim, for 2 mana more you get to discover a card in your deck, which could be a better board clear (Blizzard and Flamestrike come to mind). OR, if your opponent's board is empty you can choose a different spell that better fits your needs (Cinderstorm, Puzzle Box of Yogg-Saron, or even Conjurer's Calling to make two 8 mana minions and put a 3 mana spell in your hand). all this with a 5/5 body, and the only requirement is having the spells in your deck to cast.

Arcane Flakmage is another card which clears the board. although it's a 2 mana card, you'll probaby end up costing you 5 to play it + a secret on a turn. not only is this superior at clearing an early board, it's effect is repeatable as long as you play more secrets. it's also a RARE card, with no deck building requirements, other than having secrets.

Reno's effect isn't even close to being "Legendary", it's closer to a rare card, or an epic AT BEST, even when ignoring the no duplicates requirement.