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As the source material comes from a renewable resource it is "climate neutral" meaning that the source plants took up just as much CO2 equivalents as the burning of the biogas creates.
Of course you can start arguing that leaking methane (e.g. before burning it) has an higher impact on the greenhouse effect as methane is more potent than CO2 (in the short term, but it breaks down relatively quickly), but usually a waste material is used that would otherwise produce just as much Methane/CO2 by rotting somewhere unused.
Thus by utilizing a waste material and thus offsetting the use of other likely non-renewable carbon energy sources, it becomes overall Eco-friendly.
But of course if highly subsidized maize is just produced to be digested for biogas, that becomes an entirely different story, with likely overall negative impact.