I personally prefer the good old number box but they have their problems and you actually have to read each and ever box to see what the state is, with sliders and knobs we can see the value of a great many controls at a glance.

Some newer synths do this where it makes sense. e.g. in Phase Plant the wavetable frame is a number, since wavetable positions are discrete values from 1 to 256.

Ultimately I see two problems though,

1. sometimes the number doesn't matter or make sense at all. A good example is a macro knob. The value is somewhere between "0" or "1", and synths do let you set it manually (since this is how recorded automation works), but a macro slider doesn't make too much sense IMO.

2. lots of controls deal with logarithmic values. Anything that corresponds to a frequency is going to need finer control when you're tweaking values below 500Hz vs changing a value between 10000Hz and 10500Hz. Knobs mask this pretty well. I'm sure you could build a slider that dealt with this, but a number box would be very weird since you'd want the scroll step to be much smaller at lower values.