Liquid fusion is a non toxic single pack zero rise urethane. This style of adhesive is used in other industries such as construction to glue unseasoned / wet timber or in boat building to bond FRP to one another for a waterproof flexible bond. The mechanical strength of this adhesive is incredible!!.
Application for fly tying include - Used on your foundation of thread to really prevent spinning of materials ( Crab flies. Poppers, Deerhair etc), laminate materials such as foam, for gluing eyes that may otherwise react with hydrocarbon based glues ( eyes wont melt with liquid fusion) and repair flies or push in areas that need a void filled that UV resin may not cure in.
Liquid fusion is waterproof ( look for that wording not water resistant) and will always remain flexible. For super durable flies, it's very hard to beat.
Truth is, once you start to see a use for this adhesive, you will find an application in many things.