The problem I see with specifically focusing on the hip flexors is that most people have hip flexors that are way too tight as it is.
Most people do abdominal work as it is, on top of the numerous drills and other activities that work the hip flexors. Now, the only way in which I can see people not getting enough hip flexor work is through 2 scenarios:
1. They specifically try to exclude it in their program (some people have gone too far this way)
What I think is more likely is this:
2. They do current exercises with poor/inadequate technique and do not appropriately load the targeted musculature. I have been guilty of this at times, as have many others in my experience. It is easy to get more focused on doing volume and just doing the exercises versus doing them with good technique. If you are doing a variety of abdominal work and doing the movements with good technique (hanging leg raises, deadbugs, sprinter sit-ups, etc.) and do sprint drills with good technique, you will almost always have plenty of hip flexor work and additional will likely only cause problems.