Private operator run buses from Delhi to Nainital are rarely punctual, condition of bus varies from being excellent on some days and outright rickety on other days.
On the other hand, Uttarakhand Transport is more reliable. However, Uttarakhand Transport does not operate Volvo bus from Delhi to Nainital (@amrit_uttarakhand please note) but only deluxe bus which starts at 8:30 PM and 9:00 PM from ISBT Anand Vihar. There are Volvo buses of Uttarakhand Transport that start at the same time but run only till Haldwani. If you wish to travel by Volvo to Naintal, you need to get down at Haldwani and change bus.