The difference is that instances have self determination. On Reddit all subreddits have to operate under the umbrella of Reddit whether it's good for them or not. Here communities can go to an instance they're aligned with. For example there's a very popular German instance that has all the topics in German for a German audience. In the case of mental health, the ideal situation is thata non profit with expert guidance could create their own instance with communities lead by people who actually know what they're doing. On Reddit there's only one sub that can have a name whereas on lemmy you could have a bunch of mental health communities under someinstitute.org or something.