I've used vcs[0] to make contact sheets from videos. It has a nice feature that if it detects the frame is blacked out, it will shift the capture time so every frame on the sheet has something to look at. Easy syntax to set capture at every delta or capture x number of frames evenly distanced, columns per row, ignore a set length of end time, and frame size.
This sounds like a more robust tool, however I wanted to keep our external tooling to a minimum. The contact sheet generation sits as part of our transcoding pipeline for uploaded media which already uses ffmpeg.
[0] https://p.outlyer.net/vcs