Operative is least used and least useful. For this reason I like to have a few points in the class, so I can switch to him for hack objectives (including the super useful door hack on Resort), because often nobody else will. For non-hack objectives, I actively tell operatives to change class if my team has 3 ops, and really I should be doing at 2 too.
There are generally most medics and engineers, and they are the most usefull classes with great abilities. Between them probably most medics. You will pretty much always find use as these classes, always people to revive and buff. Often I fond myself lacking weapon buff when I’m not engineer, but then I rarely have enough revives from our medics either 
It is TERRIBLY annoying when your team lacks soldiers so you run out of ammo. If your team gets steamrolled so people spawn all the time, you don’t need much ammo, and enough ammo is enough, so there’s an upper cap on his team utility as you get more soldiers. His grenades can be super useful though, especially a well timed flashbang can support a push greatly. Due to infinite ammo, he is superb at flanking and harassing. Soldiers are much less useful on FF due to so much of his offensive power coming from grenades.
I mostly run with a spec in 3 classes, one of them being operative because he requires so few abilities (I have 3 atm, but it is too much, honestly only caltrops and maybe EMP is useful for an objective pursuing operative). It allows me to almost always fill the most needed role on the team.
