I apologize for not being more clear.  

Yes, this is one advantage to some customs Android ROMs. They remove spyware, adware, calling-home functionality, and other undesirable features from the original manufacturer ROM.  
In addition to requiring fewer permission requests for these unwanted accesses, this can:
- reduce overhead (improve the speed and responsiveness while reducing network traffic),
- increase privacy, and
- reduce security vulnerabilities. 

Many custom ROMs are also open-source. 
This can: 
- allow further customization,
- enable support for unsupported or legacy hardware and software, and
- ensure the security is openly reviewed.  

It is important to note that every custom ROM is its own project, and not all will necessarily have any or all of these advantages.

***Note: be aware there are also some disadvantages or risks:***
- Custom ROMs rarely come with any kind of warranty (and may even void the manufacturer's warranty) and may or may not have support, in contrast to most manufacturer ROMs.  
- This could become critical, if a mistake is made in the ROM's code or as the user tries to install the ROM, as there is a small risk this could brick or damage a device. Without the above warranty and support, the user would be other their own (or with only the support of a forum community) to fix the problem. 
- It is further complicated by the fact that installing a custom ROM is usually less straightforward and more complex than simply updating the factory ROM, though the exact complexity will depend on the individual ROM and its support base, as well as the user's familiarity with the process.  
- There is also the risk of installing a custom ROM from an un-trusted or less-trusted source, which could be compromised and become a threat to security and privacy.  This will depend on how much you trust the ROM maker versus the manufacturer (as well as precautions taken verifying the source of the download).

I hope this clarifies the advantages (and risks) of custom Android ROMs.

Do you have any questions?
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