The chips are dimmable yes. There are a lot of different optics available, tight beam, wide beam, diffused etc. Most large optics are made of glass.
The efficiency are a bit lower when comparing total wattage, but ease of building, replacing broken LEDs, flexibility, price etc by far makes up for that.
I do help with builds, but I'm in Sweden
But for any that like I can make a total "noob" guide how to make one.