tbh in my eyes the mercanaries are very strong don't forget that you also have bought their SOLDIERS on top so i the end you get he soldiers for free with some benefits.
Yes, it is very good to receive several troops at once, the problem is that it is only at the time of purchase, as time goes by the troops they provide daily are consumed on the same day, if the game extends even further, we will end up earning 2 to 3 soldiers per day with the daily consumption of guards. And whether we like it or not, the money to buy the troops with Noah will not be enough to supply everything.