I have used a number of belt drives over the years, and generally I prefer them over a chain drive.
However, as with most things, they are not as perfect as the manufactures would have you believe.

As others have stated, it is important to have the pulley aligned in parallel. This is just the starting point.

The typical clutch hub is supported by a bearing inboard that is close to the gearbox sprocket. The idea is to have the center of pull from the primary drive as close as possible to a bearing supported shaft. This works fairly well with a relatively narrow single or double row primary chain. Later models with a cast inner primary have an additional bearing to support the main shaft.

Most belt drives actually have a belt center that is outboard of the clutch hub bearing, depending on the belt width, it can vary a lot. This imparts a bending motion to the main shaft.
The main shaft flexes as the motor torque is varied, which changes the pulley alignments. During heavy acceleration, the main shaft flexes towards the engine sprocket. This results in the belt tracking trying to track off the front pulley.
The front sprocket alignment guide is used to hold the belt on the front pulley during these events. Yes, it does wear out the side of the belt, but it doesn't come off. I think some manufactures use a guide that screws on the outside of the pully.

The engine sprocket shaft can also flex, depending on the design.
In high HP applications, both the engine output shaft and the transmission output shaft have outboard support bearings to keep any drive system inside supported shafts.

In practicality, belt drive have been around for many successful years.
if you are looking for a belt drive for your Duo Glide, an 8mm belt is as much as you need,if it is standard.
As previously mentioned, make sure your engine shaft seal has it's spring towards the crankshaft, and be aware of wet sumping if it happens.