Except that with a raised bed you don't have water potentially also draining into the bed from the sides, which you may have with a sunken bed. Some people call the excessive wetness in a bed dug into clay the "bath tub effect". I know of someone who did this and had problems with plants because of the poor drainage.
Why not dig a test hole, pour water into it, or do it before it rains a lot, and see how it drains? If it doesn't drain well then maybe dig the bed less deep and raise part of it. Arborists no longer recommend backfilling tree planting holes with amended soil because of problems with water movement (either way) through the soil interfaces between different textured soil, which is pretty much the same effect. The compost or other organic material in the mixture will also continue to decompose and create a depression.
