[noun] + も同然だ [verb た-form] + も同然だ [verb ない-form] + も同然だ [noun] + も同然の + [noun]
も同然だ means "is as good as", "is practically": it is not literally so, but it might as well be. It goes straight after a noun, or after a verb in the た-form or ない-form. Before another noun it becomes も同然の.
With a た-form, the action has not happened yet but the result is settled. With a ない-form, what little did happen does not count.
も同然だ is a firm judgement and sounds a little stiff. In conversation みたいなものだ or とおなじ同じだ says the same more lightly. も同然だ is not a guess about how things look, which is what みたいだ and ようだ do.