A major point of the Chainlock would be invisible scouting. I have the following preponderances:
1. Would plot triggers magically teleport my party across the world into a flatfooted encounter? (Bad)
2. Can I actually converse through my familiar with the pact invocation? (Awesome flavor sauce!) I can be on the beach sipping Flaming Fire Drake shots, out of harms way
3. Can spells be cast through the familiar as intended? (The possibilities...shhh!)
4. Can I actually communicate telepathically through the familiar (Is this even a GOOlock feature?
5. Can my Chainlock familiar really explore the map (past the normal familiar distance of control) as intended by 5e?
6. Do Familiars get flight speed as intended?
Having a an extended range invisible scouting pet / ventriloquist prop could be very flavorful and strategic, so depending on how well it is implemented will make it worth it or not.
If we managed level 15 ...then Hold Monster. It's all in the RP and execution.