The Exec’s base damage is higher than those weapons and on headshot as well. It’s risk vs reward. The Exec has horde clear, elite clear, armour cleave, a little bit of monster damage, crit chance on heavies and l3, good range (especially on the heavy), easy horizontal and vertical headshot angles and decent bodyshot damage on top, which all comes at the cost of mobility and defense as a trade off.
The Rapier on the other hand doesn’t have the Exec’s bodyshot damage, its cleave, its reach, its crit chance etc. Instead the Rapier has higher mobility, more (but weaker) pushes, a better inner-angle block cost modifier and the pistol special. I believe (this depends on the career though) the Rapier has higher monster and berserker dps, while the Exec has higher armour and superarmour dps. Vs infantry it probably goes to WHC with Flense (except vs Marauders) but otherwise to the Exec, and Flense is clearly busted anyway.
The Billhook is somewhat op.
The Exec can be risky, but because of its cleave it can also be quite safe.