Tachiyomi is just a comic reader, but it can aggregate from A LOT of sources, both local files and websites. Each online source (mangadex, tapas) requires a plug-in to work. Those plugins need maintenance and workarounds for stuff like cloudflare blocks.
Needless to say a lot of content hosts aren’t fans of a client that pulls content without the user ever seeing an ad or something, so these plug-ins tend to break.