A too high acquisition time will give issues when the analog input approch the V(ref+)
A too low acquisition time will give issues when the analog input approch the zero
Where do you get this from?

The aquisition time is the MINIMUM amount of time you have to wait for the holding capacitor (and it's RC network) to fully charge/discharge to the voltage level you're sampling.

If you set the acquisition time too low then you won't have allowed enough time for the charge on the capacitor to reach the desired level, and you'll get an incorrect conversion. It doesn't matter so much if the input is near Vref or zero, it'll be wrong.

If you're measuring a DC voltage, there is no "too high" an aquisition time.