Lack of even remotely niche content (aside from Linux and infosec content)
Generally very pessimistic userbase
Lacks polish and features in many areas
Currently trends towards extremist echo chambers - the fact that .ml (an instance known for banning criticism of violent, racist, authoritarian governments) is one of the biggest instances, is a good example of this.
tends to be extremely hostile to any sort of monitization, regardless of the quality or cost to produce content
It absolutely can, but doesn't always. For example, Gamer's Nexus is well respected for their thorough and unbiased research and journalism. It would be extremely difficult for them to do so without ads and merch sales, as any products reviewed must be purchased, testing equipment needs to bought, and experts need to be hired to use said equipment. Until capitalism ceases to exist, most people who make stuff will need to find a way to fund their work, from paint brushes to high-end testing equipment. If we can't accept this, we will rarely get creators willing to provide quality content, and what we do get will be biased towards those with the money to burn.
Unfortunately, while it only takes one user to make these communities, there is rarely enough to maintain activity and esspecially not discussion on them. Even fairly large niches, such as Dota, rarely get more than a post or two a month, and no meaningful discussion despite it being a game that emphasizes theorycrafting so heavily.