This makes prowess best used in decks with few creatures but multiple cheap spells that also replace themselves by drawing additional cards

Aang and La, Ocean's Fury Abuelo, Ancestral Echo Accursed Spirit Adamaro, First to Desire Ageless Guardian Agrus Kos, Eternal Soldier Agrus Kos, Spirit of Justice Akuta, Born of Ash Anaba Ancestor Anafenza, Kin-Tree Spirit Anafenza, Unyielding Lineage Angelic Curator Angelic Page Animus of Night's Reach Ao, the Dawn Sky Apothecary Geist Arashi, the Sky Asunder Archive Haunt Ardbert, Warrior of Darkness Ascendant Spirit Ashen Monstrosity Ashen-Skin Zubera Atsushi, the Blazing Sky Augusta, Order Returned Auron, Venerated Guardian Aurora Eidolon Ayumi, the Last Visitor Azorius Herald Baboon Spirit Balefire Liege Baleful Eidolon Banshee Banshee of the Dread Choir Basilica Bell-Haunt Battleground Geist Beckoning Will-o'-Wisp Belfry Spirit Bell Borca, Spectral Sergeant Bellowing Fiend Benevolent Ancestor Benevolent Geist Benevolent River Spirit Betor, Ancestor's Voice Betor, Kin to All Bile Urchin Billowing Shriekmass Binding Geist Blademane Baku Blessed Spirits Blinking Spirit Blood Age General Bloodghast Bloodied Ghost Blossom Prancer Body of Jukai Bogardan Firefiend Bogwater Lumaret Bounteous Kirin Brago, King Eternal Breathless Knight Briarknit Kami Brine Comber Brutal Deceiver Burning-Eye Zubera Burr Grafter Bygone Bishop Callous Deceiver Carven Caryatid Celestial Crusader Celestial Kirin Celestial Messenger Cemetery Illuminator

Relegating Ambush solely to blocking and making it cost four mana just makes it unplayable for all but the most niche of battlefield states
Starter decks suit newcomers, while booster boxes appeal to serious players and collectors
When one half of a card is pretty much the Eternal staple Crucible of Worlds, its hard for anything to get in the way of its victory, even a bad second half