PDB fires from an 8 round magazine, so even if in a sustained battle against 20 zombies the compound crossbow is faster, the PDB fires its first 8 shots faster than the compound fires its first 8 shots.
comparing the stats:
"name": "pneumatic bolt driver",
"description": "An eight-shot, revolving barrel, pneumatic bolt driver handcrafted from scrap. It is very quiet and deadly.",
"skill": "rifle",
"ammo": "bolt",
"weight": 3110,
"volume": 12,
"ranged_damage": 20,
"range": 20,
"dispersion": 30,
"durability": 7,
"clip_size": 8,
"reload": 500,
"loudness": 22,
versus
"name": "compound crossbow",
"description": "A high-power crossbow with shaped cams and extra cables for high-power shots. Takes less time to reload due to a modern string-pulling system.",
"skill": "rifle",
"ammo": "bolt",
"weight": 3000,
"volume": 7,
"ranged_damage": 28,
"range": 22,
"loudness": 12,
"dispersion": 200,
"durability": 6,
"clip_size": 1,
"reload": 300,
For a skilled survivor making precise shots at a move cost of ~200 (or 90 on a follow-up shot), the PDB empties its magazine in 830 moves. The same 8 shots take 3700 moves for the compound crossbow. It’s not until the 23rd shot that he lower reload time of the compound crossbow finally over comes the magazine advantage of the PDB.
The compound crossbow is +2 range, +8 damage, -10 loudness, but +170 dispersion over the PDB. At the skill levels when you have either weapon, the survivor is probably at Rifles 4, Marksmanship 4 or better and isn’t missing with either weapon and the damage is lost in the noise.
The compound crossbow is in theory easier to make - Fabrication 7, Mechanics 5 versus Mechanics 8 for the PDB, but it’s book learn at Fabrication 8 so not really.
I personally think the balance between bows and crossbows is fine as it is. I wouldn’t mind seeing a mild nerf to bows but I think they’re acceptably balanced. Oh, except the pistol crossbow and repeating crossbows - those need help.