There are certain downsides to Multithreading. These are:

1. Difficult to Debug: Multithreading code is difficult to

debug in case of an issue.

2. Difficult to manage concurrency: Due to multiple threads,

we may experience different kinds of issues.

3. Difficulty of porting code: It is difficult to convert existing

single threaded code into multi-threading code.

4. Deadlocks: In case of multi-threading we can experience

deadlocks in threads that are waiting for same resource.

https://www.javatpoint.com/benefits-of-multithreading