As much as I like the cards and concepts here, it is below average for Commander
His use will really depend on the meta, and right now it seems like Aggro and Midrange decks are in
Sure, it costs five mana to do which is not cost efficient, but this just means you are not doing it till it is really good to do so or you have little else
On the other hand, if I wanted to wipe your board with Extinguish All Hope, your Summon: Bahamut laughes in my face
While Ratchet Bomb isnt a Modern staple, it does see play, often in colorless decks lacking removal options, as a Karn, the Great Creator tutor target or in mono-colored decks that cant easily power up Engineered Explosives as an answer to Urza's Saga tokens
Cards like Savage Ventmaw (plus a mana dork), Grand Warlord Radha, or a ton of mana dorks convert this into easy infinite extra combats