To be exact, it's not sub for sub that hurts.. it's inactive readers.
Basically, you want a high "engagement ratio." That is, you want a good ratio of people who view, Like and comment, versus the number of total subscribers. If you have a lot of active readers who do those things, that increases your chance at making it to the front page via Popular/ Trending.
But if you have a ton of inactive readers who aren't Liking/commenting or even reading, that makes it extremely difficult to make it to Popular/Trending.
Soooo if you gain a ton of subs via sub for sub, and you don't read each other's comics, that's going to make your engagement ratio go down.