Advantages:

• The screen is protected when the phone is closed
• It allows for a larger screen while maintaining portability

Disadvantages:

• The screen is plastic, instead of glass. Meaning it is very easy to scratch
• The screen has a hump where the hinge is
• The hinge can get dirt in it, at which point you start developing lots of problems
• They are very expensive

I personally buy mid-budget phones, which means these aren’t even a consideration. But, I probably wouldn’t buy one even at the same price due to the plastic screens being very easy to scratch and a larger screen not having much value for me. But, really it’s up to you what you value in your phone.

The screen is plastic, instead of glass. Meaning it is very easy to scratch

It’s actually a specially made glass that is thin and flexible https://www.schott.com/en-us/expertise/applications/schott-utg-in-foldable-displays. You might be thinking of the factory installed screen protectors on most (all?) foldable phones that is made of plastic.

I had a Galaxy ZFold 4 and there were lots of things I liked about it. The larger screen really does make a big difference for consuming content and looking at/signing documents. I ended up going back to a traditional style phone because I realized that I spent a lot of time thinking about general fragility of my phone in a way that I hadn’t before. Cases are significantly more expensive and require adhesives to stay attached. I had a few in warranty repairs from debris in the hinge as well.