According to my experience and turntables that I measured, direct drive's wow&flutter performance isn't better than belt drive. Basic and very cheap direct drives can be very good up to a certain point but after that even high end direct drives can not improve w&f figures much further. Another strange thing that some modern direct drives offer worse performance than old, vintage ones. The best w&f results can be achieved with top belt drives according to my experience. I haven’t measure modern idlers but old ones are not far behind of direct drives.
Belt drive’s problem is belt, direct drive and idler’s problem is not being able to separate motor from the plinth (there are some exceptions like Esoteric T1). Direct drive motors have another disadvantage like turning at platter speed (33.3rpm). Idler's disadvantage is noise because of idler mechanism.
IMHO/IME there are more important factors than drive type which makes a turntable sound good or bad. One of those factors is high mass platter. A high mass platter (preferably over 20kg) is the key factor for a good sounding turntable and helps solving problems introduced by drive type (BD, DD or idler). I believe implementation and how good a turntable is built is more important than drive type.
Keeping steady speed at 33.3rpm and w&f performance are two different things. Another very important problem for all drive types is aggressively trying to keep 33.3rpm speed by control electronics. That aggressive control may achieve 33.3rpm average but for the cost of increased w&f. Piano and violin sounds are very sensitive to w&f. It's a good idea to check how piano and violin sound when evaluating a turntable.