I believe early mercenary play (early feudal) is a jebait UNLESS you have fast castle aspirations. Only land maps are being considered in my analysis.
Berries are the slowest form of food in the game for most factions to include byzantine. However byz get a perk that allows them to get 50% of foraged Berries as oil which on paper sounds Great!!
But here are the current limitations to the paper mache.
For simplicity oil is used in batches however that simplicity also adds inflexibility since early you cannot readily and consistently produce oil in batches of 400+.
While berries are readily available in base, berries are slow and inefficient source of food due to lots of walk time and lowest gather rate in game; and you only get 6 bushes in base which amounts to 750-900 oil (the higher yield is a bonus of a feudal landmark).
And because berries are slow and not abundant you not only forfeit faster food but also your age timing and unit production/overall macro given how many more villagers you’ll redistribute on berries.
to use the mercenary mech you need build a 150 wood production building and spend 100 of your oil to research a contract. Then after all that you get 1 to 2 batches max for the in base berries (2 batches only if you save foraging until after you build the extra oil landmark and make use of a contract that allows you to spend 400 oil per batch).
Next to keep this inefficient food production going you now how to migrate to your next berries as if you were delhi except Much much much sooner and have your villagers exposed to raids collecting that slow food???
All this you’re doing while managing the extra APM for cistern connections standard macro in hopes of garnering leverage over your foe???
I think intentional mercenary play should be reserved for when berry food is the last viable food available and/or during mandatory farm transition and late game trading.