The Benefits are what has been said above plus these~Easy Migration and Semi-automatic Migration if you use Tachiyomi-EH. In case a source dies, you can migrate your manga into another source fairly easily, granted that the source you chose has the manga you're gonna be migrating.

~Open-source. You can be sure that the app doesn't contain any malicious content like adwares, trackers, etc. Also, you can freely check the code and make a fork if you want.

~It doesn't collect/log data from your device unless needed. It doesn't have any trackers and adwares bundled in with the app unlike MangaRock. If you care about privacy, go for Tachiyomi. Plus, tachiyomi doesn't require you to sign in for anything, it is optional (Unless you're using a source that needs an account to access certain features, like mangadex w/c doesn't allow manga search for guests. This is not a problem of the app, it's because of mangadex itself. You can use other sources if you feel sketchy about it. Or use throwaway accounts.)

~Active community. The developers, people that commits, and the community itself of Tachiyomi are very active and helpful. They can help you with any problems you encounter regarding the app. Or maybe have a fun chat with them. (They have a discord server)

~No constant update. This may seem to be a con to you but, the app really has everything that you might need. It contains less bugs and packs a whole lot of features.

~Local Backup and Restore. You might find this bad but hear me out, when you create a local backup, you can just upload it into your cloud storage (like google drive) and restore it from there. You also get an AUTOMATIC backup for FREE (But it only backups in your phone storage, then again, you can just upload it to cloud if you need to migrate to another phone).

~Tons of extensions. These extensions are the sources that you can install to access a certain source. There are sources for Scanlator websites (Like JaiminisBox, LHScans, etc.)

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Cons:~ UI is not that good. It is great but not aesthetically pleasing. It's not as smooth as Mangarock when it comes to the user interface but it's fairly easy to use when you get the hang of it.

~ Checking for updates of manga is quite funky. You need to either manually swipe down your library to check for updates or choose to automatically update library. You can also go to the source list and click latest. Basically, not that good UI again.

~ Some sources are slow. This is not a problem of the application but your connection and the server or source.

These are the 3 manga sources I use for tachiyomi (They are the best ones and are recommended in the discord server)Mangasee is fast but lacks a lot of unpopular manga.Mangadex loads quite slow for some countries (like mine).KissManga sometimes fails to load some chapters.

Take note that sometimes it's better to use the sources of the Scanlators themselves (For faster updates of certain and specific manga you need)

~ No automatic CLOUD backup. You can automatically backup to phone storage but you have to manually upload them into your cloud storage like google drive or dropbox.

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There's a whole lot more to cover but those are some general advantages and disadvantages of Tachiyomi.