Both contents and the publishing model matters. Let me first raise some points about the latter.
Academia Letters is not open access, which should be the future of academic publishing — not locking knowledge behind corporate interests. That’s precisely the problem with the current academic publishing model and Academia Letters does not change that — with the latter you may not pay with money but you pay by handing over your data by having to register to the service to read the articles. If you want to disseminate your work broadly and freely, you can use any number of free tools (https://ssrn.com, Medium, etc.) that do not require your readers to register to the service.
Then to the content. Academia Letters is clearly modeled after academic journals, see: https://www.academia.edu/letters/submission_faq. I call it a fake journal because, despite the superficial similarity to traditional academic journals, it is not governed by an academic community. This matters because academic knowledge lives in an academic community and if you cut out the community, it’s not anymore academic knowledge you publish. You may think that the ‘quality’ control offered by Academia Letters makes publications somehow more prestigious, but I am afraid this is not the case for anyone who actually bothers to check out the information about the editorial process.
Finally, Academia Letters can be seen to try exploit academics by sending out seemingly algorithmically generated requests to perform reviews. I agree that different people may have different views about this — you may be happy do such volunteering for a private company — I am not.