Most of these points aren't specific to FBA and apply to all 3rd party sellers on Amazon.
One big advantage of FBA is prime shipping. I usually pick the items wiht prime shipping over other 3rd party options even if it's 20% more expensive.
I used to do the same thing and filter search results by prime for the shipping benefits. However since they've started using UPS Sure Post with hand off to the USPS I've been having nothing but trouble getting my packages. I'm just about done with Amazon Prime due to it.
I've had similar experience with Prime shipped packages, and it's quite disappointing. I finally canceled my membership after their price hike to start a music service I'll never use.