Any cover that a Czech Hedgehog provides to infantry is marginal and the benefits of deploying them outweigh the risks. Here an image of what getting cover from Hedhegogs would look like. Note that the Hedgehog itself is is about 3/4 the size of a standing man and when prone it can only effectively cover about 3-4 people.

The primary purpose of using hedgehogs is to prevent tanks from being fully effective due to the fact that a tank that attempts to plow through one will end up rolling on top of it and possibly be disabled. Their design also ensures that explosions that make it roll around will still make it functional.

Tanks are much better for getting rid of entrenched defenses than infantry are. So while it is true that the hedgehogs provided cover to some extent to infantry, a tank would have to be more careful navigating the beaches of D-Day and thus be a more susceptible target to anti-tank weapons.

I think everyone else has the practicalities sorted out.

Regardless of the actual cover afforded, however the beach scenes you mention are pretty close to what actually happened. A large number of engineers were landed with the assault waves with instructions to blow gaps in the obstacle belt. Their after-battle reports contain numerous accounts of having to get sheltering troops to move away from obstacles that were to be blown up. Or having arriving boat teams cluster around obstacles that were already rigged with explosives.

Essentially, to an individual under fire in an otherwise barren landscape the tendency is to find anything that looks like it might provide even the slightest bit of cover.

Joe Balkoski, Omaha Beach