What is the benefit of spoilerons compared to ailerons?
Two reasons often-cited for having spoilerons:


better roll control (especially during stall)
doesn't take up space for flaps


Because of their location, they don't need to take up space which could be used for flaps. For example, on the MU-2, the primary reason for using spoilerons was to allow for full-span flaps at the rear, lowering clean wing area while still having acceptable stall speeds in the takeoff/landing configurations (and therefore shorter landing distances). This is one of the reasons for it to takeoff from runways less than 3000 feet, while having a cruise speed of up to 300 knots.

However, spoilerons have the disadvantage of killing lift, which can be deadly in situations such as a one-engine-out situation in a twin. To compensate for this, usually minimal constant spoiler use is recommended, and mostly rudder is used if an engine fails. If available, aileron trim (despite the name, there aren't any "real" ailerons on such an aircraft) is used to reduce drag if long-term use is expected.

Spoilerons tend to have less of a difference in roll rates at low and high speeds, compared to ailerons -- this could either be considered an advantage or a disadvantage, depending on the situation and whether a pilot is used to one type of roll control or another. This can negatively affect the roll control feel of the aircraft though, as it might feel slightly sluggish at higher airspeeds. They also tend not to provide much (if any) feedback to the pilot, which is a disadvantage.

Note that most large airliners use both spoilerons and ailerons for roll control, to be able to still have an aileron while supplementing large control inputs with the spoilers.