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Relentless Assault creates an additional combat and main phase only if it resolves during a main phase

Gone are the days people would run commanders just because they were the colours they wanted access to Card advantage every top commander provides card advantage in some way, whether its literally drawing cards, card selection via scrying or similar, bringing cards back from the graveyard, tutoring, or even producing permanents such as tokens

You actually get more value running it in Pioneer, Historic, or other formats that allow up to four copies of a valid card in the deck

It may be superseded by a later control-change effect