If you use the workflow of an immutable system on a traditional one, you have almost all the disadvantages of the first and pretty much no advantages of the second.

The "immutability" (you can still change stuff) is the wrong thing to look at.
I prefer the term "image based", that fits better. Everytime you update your image system for example, it gets "pulled" or compared to the original image.

On a traditional one, you have your original image you once downloaded, and that's it.
Over time, it will still change due to updates and stuff. An immutable is basically a "fresh install" every time.

Most immutables use layering, so you still pull the original image, but after that some stuff gets changed.
It is generally strongly discuraged to install stuff (like GIMP and so on) directly. It should only be the last option, like for drivers.

But even when you directly install, you don't use all the benefits. The OS is still rebuilt every time and package drift barely happens.

And, back to the beginning, the pros and cons.
It's like with PDFs. Yeah, it sucks that you can't edit them. But that's what they're built for. They can't be tampered with, but therefore they look the same on every device and you don't have to worry about fonts, formatting or symbols.

And on immutables it's the same: some stuff is a bit more different/ complicated for some, but at the same time, they're less buggy, more secure, offer instant rollbacks, can be customized and rolled out super quickly, and much more. Read my other comment for more information, including customization by building custom images :)