Rare has said they want different weapons to have different uses.
The cutlass uses no ammo, has no reload and is short range (besides the dash/thrust attack).
Pistol has a nice mix of range and damage, and a knockback effect too.
Blunderbus is extreme close range and basically worthless if they are more than an arms reach away from you.
Scoped rifle has great range, flat trajectory but is very slow to aim with in close quarters.
So overall each weapon has their use.
If someone is coming at you with a blunderbus my advice is to simply pull out your pistol and shoot them to try and knock them back... optionally use side-strafe and use cover, like the masts, rocks, the wheel for raising the anchor etc.
The blunderbus really needs all the shots to hit in order to kill you so if any of the pellets are stopped from hitting a mast or other forms of cover you stand a good chance to go forward and hack them to pieces before they can reload it.
Personally I prefer the pistol as my go-to gun.
Another thing to note is that we seem to be able to carry every weapon at the same time, even though keybinds only let us bind Weapon 1 and Weapon 2.
Maybe this is a bug in the beta though.
I would prefer it if we could only carry one gun in addition to the saber so you would have to choose what to carry... pistol OR rifle OR blunderbus, rather than carrying all 3 + saber at same time.
That way players would have to make a tactical choice on what to use depending on what they are doing.
Are you about to board a ship? Grab the blunderbus!
Are you covering your friends from shark attack? Grab the rifle!
Are you not entirely sure what to expect? Grab the pistol!
Carrying all at the same time feels a bit like cheating to me... but hey, that's my opinion ;)