This can generally be divided into two types. Each with its own pros and cons:
Hard luggage
The typical hard cases. These have the biggest advantage that they can be locked. Also, when packing, these are often easier to divide into smaller compartments to keep an overview. A disadvantage may be that they become poorly lockable when they have endured an accident. Safety can also be a concern. Mainly in accidents on unpaved roads where the hard cases can come onto the rider’s legs, with all the consequences that entails.
Soft luggage
These are available in all shapes and sizes. From simple side bags, roll bags tot systems that do not require a rack. An advantage of soft luggage is the flexibility it offers and the safety in the event of an accident. The risk of physical injury from soft luggage is considerably smaller. Also the problem of not being able to close them after an accident is much smaller. However, a disadvantage is that soft luggage cannot be locked. There are chains and special backpacking nets available, but it is not a waterproof system. Depending on the type of soft luggage, packing can also be a bit of a challenge.
My choice
My choice fell on a combination of a Mosko Moto Reckless R80 and various bags from Turkana. The modular properties, a bit of safety during off-road riding, and the fact that I can easily switch the bags between different motorcycles made me prefer this. For me personally, not being able to lock is was not really an issue. When I’m on the road, I never leave my motorcycle, and thus the luggage, out of sight.
EN 
NL