It's important that you understand that the quality of the motors INSIDE the EF lenses vary considerably.
There are lists on REDUSER tested performance on a large number of lenses, where users have rated the level of performance they get from different lenses. Broadly speaking, L series with USM motors and Sigma ART are really good!
FOCUS is extremely accurate. There is slightly more 'stepping' than you'd get with a 'real' lens motor, but it is extremely usable. It's a small price to pay for the massive bump in capability!
We have found that if the internal motors are racked very fast over a period of time they can occasionally lose their end points. Again this varies according to which lens you are using. If you have found this to be the case a quick power cycle or recalibration will solve.
IRIS generally has noisier motors, and does move in steps. On some lenses it is 1/3 stop steps, others go more fine. It's best to think of Iris adjustment as something to do between takes - there may be lenses which do silky smooth iris (now, or released in the future), but that's in the hands of the lens manufacturers! (.. we send the lens very smooth data, they do the rest).