Any faction that has invested in TD while not facing tanks is in a disadvantage. USF suffer less than other factions since they combine powerful infantry and vehicles crews.
First half of that statement is completely incorrect.
In fact, getting a counter for a unit that haven't arrived yet is what's called "anticipation" and it has dual benefit.
First, if opponent does get the unit, you already have a counter for it on the field and opponents shock value is reduced to zero.
Second, if your opponent is aware of the unit, he is less likely to build what he planned to build, again, nullifying shock value of stronger unit and if he gets the unit anyway, he is going to be much more conservative with its use, not diving for securing a wipe.
You basically exchange possible shock value of your own tank into defensive game and deterring opponent from comitting to his armor.
So no, there is no disadvantage of investing in TD before you see opponents tank.
There is however a disadvantage of getting multiple before opponent gets any armor.
Vehicle crews are hardly relevant here, the cost of TD however is since you'll have only defensive armor presence and will have to rely on infantry/lights preserved until that time and strength of rifles can be completely nullified by one well micro'd ost sniper.