After you chose your side at the end of Act II do the rival factions send Assassins after you like in NV? My next quest n the MQ is Hunter/Hunted

If you side against the BoS their Vertibirds will try to kill you, I don't think they'll actively be hunting for you but you're kill on sight to them.

Given the sheer number of vertibirds that are constantly in the skies after Kellogg dies...... They don't NEED to actually be hunting you, they are pretty much omnipresent....... there is ALWAYS one nearby.

They don't send assassins, but you may stumble upon their patrols and they will attack you.

yeah no assassins like Talon Company in FO3 or rival factions/hit squads in FO NV. I remember Caesar sent patrols out for me but they were hardly stealthy. I could spot the stupid banners a mile away and shoot them with my sniper rifle before they were close enough to even spot me heh.

But I figure with the respawn rate being so high in FO4, who needs assassins? there is already enough people around the next corner wanting to kill you.

I would suggest however to Bethesda, next time make Raiders a bit more tactical. Instead of me walking by a building and be alerted to Raiders on the roof when someone says from above "Who's there?" maybe have them sneak around and try and ambush me from the shadows. Now that would be more exciting.