I think the biggest issue is not even the left/right or political bias of Wikipedia but rather the fact that some committee of wiki editors decide along what seem to be fairly arbitrary/subjective lines that some sources are reliable and others aren't.
And then those claims make their way into Wikipedia where they inevitably (even though they shouldn't) are relied upon by students, politicians, journalists, who then perpetuate the claim.
It's not a committee, it's not arbitrary, and "arbitrary" and "subjective" mean two very different things.
Reliability from a fact-checking perspective is a pretty specific thing, and a thing that is vital to Wikipedia as an open-source, anyone-can-edit encyclopedia. This can correlate with political views in particular times and places, but does not broadly correlate with either left or right. E.g., after the Russian revolution, we saw the left using Pravda as a vehicle to "indoctrinate" and "encourage unity of thought". [1] But a significant part of the current US right has frequently taken the approach of "flooding the zone with shit" [2].
And then those claims make their way into Wikipedia where they inevitably (even though they shouldn't) are relied upon by students, politicians, journalists, who then perpetuate the claim.
https://xkcd.com/978/