Alchemer guns have a small clip, short range, slow bullet speed, and (it seems like) slow reload time. What are the pros to them?
The only unique property of the alchemer is their charge attack, a bullet that shatters into smaller bullets. But on a more general note, make sure you see the 5* versions of the guns in action before making comparisons - Alchemer bullets aren't slow; when upgraded, they're one of the fastest traveling bullets in the game!
This is most obvious at the 5* versions, where a well placed alchemer charge shot can shatter within the body of a larger enemy, causing all the resultant bullets to instantly hit the same monster. This means that the alchemer possesses some of the best burst DPS available, though it may be hard to pull this off consistently.
The Shadow Driver series was also, until recently, the best pure shadow gun (Biohazard has slower bullets and a much less useful charge attack).
Finally, the reload time is constant across guns - the difference is the clip size. This can be worked around, however - just pause between each shot, and you'll never have to reload.
Another bonus for newbies is that you can get alchemers for cheap in the auction house, like 1000 crowns. Alchemers were my first exploration into shadow (Shadowtech Alchemer) and elemental (All other Alchemers) damage types. Also, if you need piercing for newbies, I suggest getting a flourish, also around 1000 crowns in the Auction House.