There are a few. Tachiyomi and Crunchyroll Manga are worth checking out. Tachiyomi is known for its customization options, and Crunchyroll Manga offers simulpubs of ongoing series. It depends on your specific preferences though.
Well, legal manga apps usually have partnerships with publishers and follow copyright laws. However, there are a lot of shady apps out there that violate these laws and are definitely illegal.
Manga Rock is a popular one. It has a large collection of manga novels and allows you to access some for free. Another good app is Manga Reader. It has a simple interface and offers free access to a decent amount of content.
Webtoon is an app that features a wide range of webcomics, some of which can be considered as manga - like graphic novels. It has a large number of free series available. The app has a unique format where the comics are presented in a long - scroll format, which is different from traditional manga layout. It's very popular among fans of digital comics and is a great place to discover new and interesting free manga - inspired graphic novels.
In most cases, they aren't bad. But some poorly developed manga apps might have issues like battery drain, compatibility problems, or security vulnerabilities. So, it's important to choose well-known and reviewed apps.