The Albins ST6-M has a roller barrel selector mechanism which is pretty much the gold standard for gear change mechanisms in motorsports. The following video does a great job demonstrating how it works on EX-Jordan F1 gearbox. The gear change section starts at 5:15.
Advantages
Few moving parts.
Excellent reliability.
Direct linkage between the selector forks provides optimally timed movement of each gear’s dog ring.
Simplifies the movement of the shift actuator into linear motion.
Ability to withstand loads generated by very fast gear changes .
Ability to withstand very aggressive shifting (lots of force on the gear change mechanism).
The clutch is only used when moving from neutral to first gear or from neutral to reverse.
Disadvantages
The disadvantages primary stem from the fact that there is no synchronizing mechanism to assist in equalizing the speed of the gears.
Greater wear on the gears. Race teams view this as the cost of doing business.
Requires greater skill to downshift because the throttle must be bliped to match speed. This isn’t an issue if you have paddle shifters and a Gear Control Unit (GCU) does it for you.
Shifts are more abrupt.