Some frames have all the arms and bottom plate cut from a single piece of carbon fiber sheet, this is often referred to as a unibody design. Unibody is much lighter and makes frame assembly easier, but if you break one arm, you would have to replace the whole bottom plate and move all your motors across to a different frame, it's a lot of work. In comparison, with separate arms you can simply replace the broken arm. Been debating this for my next build. I generally find frames with replaceable arms stiffer than a unibody, because the arms are usually sandwiched between two separate plates. What's the real-world experience here?

I've crashed plenty of both designs. As for rigidity, I generally find frames with replaceable arms stiffer than a unibody, because the arms are usually sandwiched between two separate plates. But of course, arm stiffness depends on carbon fibre width and thickness too. Replaceable arms saved me so much money and downtime.

Depends on your flying style honestly. It's going to get broken, because you will crash. A lot. But as you learn how to fix, you'll figure out what parts fit well together - if you're learning, replaceable arms are definitely worth it for the repair convenience.

The weight difference isn't that significant in practice. Where cost isn't the predominant factor, carbon fibre is usually the ideal material. For racing/freestyle applications, carbon fibre is often the primary selection as it suits the 'low weight and high durability' mini-quad material criteria. I'll take the extra 10g for easy repairs any day.