Problem one: While you can hit enemies at Extra Reach range, you do not Threaten enemies at that range. You have to get close for that to happen, and that's...weird.
Problem two: Sentinel is a lot less powerful than in PnP 5e in regards to Extra Reach. Much of the appeal of having it was combining it with Polearm Master and being able to attack and stop one enemy BEFORE they got in range to attack you (if they don't have Extra Reach themselves). In BG3 the extra reach isn't used for the attack of opportunity, so the attacker will be able to hit you regardless of what weapon they have equipped.
