Cluster caltrops wording makes me think...

...'for each enemy unit within 1" of a model with cluster caltrops that Falls Back' in this phrase is the enemy unit that falls back or the model with cluster caltrops?

Yeah I think many people misread it. It's not completely clear but to me it's: if the reavers leave the combat, they release the caltrops... which actually makes sense because in real world caltrops are something you use to damage someone running after you.

So yes, they are an instrument to help reavers if you want to use them as a shooty unit and not staying in close combat.

That way they get more useful than grav-talons, you get to use every turn, if you get charged or you charge, and it discourages small unit to try to tarpit them.

it depends... if you leave combat (using caltrops) later on you won't be able to charge again... so it works more if you want them to be shooty... if you want them in close combat you probably prefer to have that attack before your enemy, not later on.