Multisite subdomain vs subfolder best for membership site

I know the debate is old but I can't find an answer anywhere to this question:

I'm building a membership site where each user gets a multi-page subsite from New Blog Templates. The entire network is SSL-encrypted, and users have no admin access anywhere. Subsite SEO is unimportant because each subsite is restricted to the registered user. There is no domain mapping.

I'd like to use a popular messaging app on the subsites that may not work with wildcard subdomains, so I'm considering building the network with subfolders. What are the benefits of using subdomains? Why do some network admins prefer subdomains?