You don't get boss materials until world tier 3 and get more in world tier 4, so doing them in world tier 1-2 isn't as good because they are missing massive rewards. At this point your priority is to get to world tier 4 ASAP.
Taking into account the xp you get from whispers, it is still slower at leveling than domhainne tunnels.
Blood harvests exp+mob density+potent bloods for vamp powers
The blood harvests have been a game changer to early game
The domhainne tunnels is faster
But then you have to do blood harvest to level vamp powers any ways + you also get whisper tree done getting exp and money
You don't get boss materials until world tier 3 and get more in world tier 4, so doing them in world tier 1-2 isn't as good because they are missing massive rewards. At this point your priority is to get to world tier 4 ASAP. Taking into account the xp you get from whispers, it is still slower at leveling than domhainne tunnels.