Here we go again 
I guess i’ve got it in the end 
So after all the trial and the new update of NS ( thanks btw ) forcing ikev2 made the tunnel work without issue
I / We just need to remember to uncheck / disable the Dynamic Routing on the unifi interface
Unifi CFG:
NethServer CFG:
Ping Working
As a side note, if i forced USG to use ikev1 the tunnel was working fine in the end