In games where you spend a lot of time in coversations it makes sense. Fallout New Vegas is one of my favorite games but if you do nothing but max Speech it turns into easy mode.
The truly horrifying combo is Battlemaster+Gloomstalker+Thief with a couple specific hand crossbows and the sharpshooter feat. All that Dex + 15 AC from Yuan-Ti scale mail they sell at last light means 4 attacks per round each and 20+ AC.
Oh, and stealth, and lockpicking. Astarion? Never heard of him.
Had to use a bonus action for something? Then you've got battlemaster ranged moves to fear, trip, or disarm. It's disgusting.
If this was written by the Devs it would seem to confirm that Deepened Pact stacking with Extra Attack is intended. Seems absolutely insane to me but what do I know.
In 5e I can take a level of warlock and at character level 5 the cantrip outdamages a heavy crossbow.
I can make a warlock 2 / bard 3 and both of their selected cantrips scale based on Character level.
You should absolutely be able to mix and match extra attack features. There isn't a reason, other than 3+ attacks is a primary fighter feature, that extra attacks shouldn't stack.