I would not say that the police "sends" mercenaries. Mercenaries just spawn behind your back immediately. They do not try to find you and hunt you down. The do not follow you through the city. Even the last two Assasin's Creed games have more sophisticated (although still quite simple) and interesting wanted system with mercenaries that try to find you and kill you.
Here we go:
A) An interview with Mateusz Tomaszkiewicz, a quest director (June 13, 2019):
Check 14:00.
Well, what we have right now is quite different from what he says:
1) The police attacks the player not only when they SEE us committing a crime. They attack each time and everywhere, even if they are not around and even if they do not see us committing a crime.
2) As far as I know, the corporate security does not attack us if we commit a crime (only if we attack them).
3) As far as I know, the gangs do not attack us if we commit a crime (only if we attack them).
4) The police (NCPD) does come after us in Pacifica. Along with MAX-TAC and some mercenaries. Not gangs.
B)
An interview with Alvin Liu from CDPR (Jul 17, 2019)
Let me repeat:
So I would compare it to The Witcher 3 where if you chopped off the head of a villager in the middle of nowhere the guards wouldn't show up out of nowhere.
It's nothing like that in the game. It's exactly the opposite. The police shows up out of nowhere.
Well, there was nothing to spoil. The answer is simple: no.
He also says some other interesting things:
Can you follow NPC in the game and see the same thing repeated ONLY a few times during 15 hours?
There are no invisible walls in the game? (BTW, I am not talking only about Badlands. E.g. huge part of Pacifica is blocked off with invisible walls. There are also many other areas, especially the ones that are close to a monorail track, that I could not explore in the main part of the city because of invisible walls).
Can we have a low or high reputation with some local gang? Can we see ourselves through reflections (beside mirrors)?
Are we getting more street cred experience for wearing certain outfits while killing?
Etc.