Official forums have lower traffic and complaints on a wide level can easily be taken down by the management as to not make the game look bad.

Reddit is Reddit. If you can even find an active Subreddit you have to contend with crappy answers, terrible search functions, and rules that often disallow many of the right posts to be made.

Fansite forums are, again, much lower traffic.

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None of those alternatives are as good as the Steam forums which have high traffic, easy search functions, a focus on troubleshooting, and a community more likely to give an actual answer than most others. The majority of PC troubleshooting happens on the Steam forums for a reason, and it's not because Steam fanboys want to help Valve's monopoly or anything like that.