> static pages
As in it won't change after you load the page.
> almost no advertising, fewer working paywalls
Indeed, haven't seen these with mastodon instances, but you never know when that will start happening.
> smaller attack surface
The instance could have been hacked, or you could have angered the admin, or you could have angered some other user who knows of a vulnerability they could leverage to send you custom JS.
The JavaScript sent by Mastodon is obfuscated, so it isn't reasonable to expect to be able to audit it and mark it as safe. You could YOLO and allowlist an JS from trusted instances of course, but that opens you up to the scenarios above.