To me it depends on the type of content. In terms of character options, spell options, etc. I am not interested in unofficial content because it has little guarantee of being meaningfully playtested, and when it isn't unbalanced or out of sync with familiar or typical 5e mechanics it often falls in a trap of being too conservatively "the kind of thing WotC would definitely publish" to be interesting. In any case there is so much official content here for the amount of characters anyone will ever actually get to build and play that I'm just not interested in searching out which unofficial options seem of equivalent worth and adding them to the mix.
But then there are monsters. The 5e Monster Manual involves pretty anemic monster design created before anyone really had a sufficient feel for how the game played, so although WotC's monster design subsequently improved, I would rate 3rd party content as better on average because a much larger percentage of it was made by people who had a feel for how monster fights go in 5e D&D as actually played.
Most importantly, there are adventures. WotC's short adventures are mostly fine (currently running my 3rd and 4th groups through Lost Mines of Phandelver and still love it as a product), but the full book affairs are (almost) all just horribly compromised by having too many cooks not really knowing what each other are doing, and probably working on pretty hard and fast deadlines. "Compromised" is the best word in general because they also seem to (presumably) be heavily reworked in response to input from the marketing and legal departments, and sometimes also a cultural sensitivity reader, and likely in response to what other teams working on other adventures are doing. Or else they just have too many authors in order to get giant books written in short spans of time. Whatever the cause the results tend to be undigestable, messy adventures full of vestigial elements from earlier versions that don't really cohere or make sense, that are just way too much work for the DM. The only real selling point of official adventures for me at this point is that, if you run them on a VTT you can google image search basically any location ever mentioned in the adventure and find a battlemap someone has made or at least picked out specifically for that location.
So ultimately I voted true, primarily because I think the major official adventures are simply no longer functional products.