Is there any disadvantage of magnet encoder?
Generally lower resolution, higher latency (magnetic orientation needs to be processed), and higher power consumption.
Magnetic encoders are a newer solution to the problem of optical encoders being fragile, and prone to issues from heat, humidity, vibrations, and dust in industrial environments. If you don’t anticipate an environment like that, there’s not really a reason to go for a magnetic encoder, as far as I’m aware. Even if you had the resolution you need, and power consumption wasn’t an issue, you’d be introducing position latency, so make sure it’s justifiable.