Generating thumbnail from only the first megabytes of a video file
Can there be a reliable answer?

No.
As with almost everything is video. The real answer is: It depends.
With mp4, the moov must be at the beginning, AND the mdat must properly interleave tracks. For example, If the there more than one megabyte of audio before the video, Then it won't work even with the moov at the start.
For TS, there must be a SPS, a PPS and and IDR in the first megabyte. That is probable, but not guaranteed.
For Mkv Interleaving matters.
The codec, codec configuration, and even bitrate will also matter.
If the first keyframe is over 1Mb, no container will work.
If the codec is using periodic inter fresh, it would probably take more that 1Mb, unless the video bitrate is very low.
Writing all the permutations would take a book.