Crossguard Lightsaber - this prevents this

If that were the case, a cortosis woven crossguard would be cheaper, easier, and safer.

1. Those are impractical. They have the ability to create specially designed metals that resist being cut by a lightsaber. It works by creating a magnetic field similar to a lightsabers and thus blocking the lightsaber from cutting it.
2. Looking closer they look more like exhaust ports, they are a jet like which would mean they likely don't even have a magnetic field to begin with.
3. The blade itself is clearly unstable. the Crystal he is using might be warped or damaged because his blade is not a solid beam but is broken up. This would help explain the Exhaust ports because its creating an excess of plasma in the blade and isn't balancing it properly, so it has to funnel it outward.

4. Darth Vader came from the side, he would have cut off Luke's hand even if he had a system like that.