score:5
To amplify the comments: yes, this is a significant risk. If the outbound flight is delayed, you may not be able to pass Singapore exit immigration before your visa term expires. If the outbound flight is cancelled, and there's no outbound light to your destination the same day, things could get thoroughly unpleasant. Here, you're risking an overstay, which will not look good if you seek to travel to Singapore in the future. If Singapore shares its immigration data with any other country, you may find that country also looking askance at a visa application from you.
Thus, the consistent advice from this webpage will be schedule an earlier departing flight. If there are multiple flights from Singapore to your destination, a few hours' difference might be enough cushion. If there is only one flight per day, choose a departure flight at least one day in advance of your visa termination.
How large a cushion you choose will depend on the severity of the consequences if something goes wrong (e.g. delayed or canceled flight) and you miss the visa deadline. Here's an example: my wife and I took a boat trip to Antarctica a few years ago. We had a multi-stop itinerary just to get to the departure port, Ushuaia, Argentina. Missing the boat departure would have been unfixable. Thus, I scheduled to arrive in Ushuaia two days before the boat was to depart. This is an extreme example, but if we'd missed the boat, we would have both missed the trip itself and we would have lost the significant paid-in-advance cost.