Nothing indicates that Missouri was nerfed.    On the question to take it or not, you must keep in mind that 750k free XP is really EXPENSIVE !! It's more than the cost to unlock Montana if I remember correctly.   Even if Missouri is a very strong ship and makes a lot of credits, you must ask yourslef if you really need it. Do you play US BB so much ? Do you already have T8 premiums that can already give a good amount of credits ? And keep in mind that contrary to T8 premiums, Missouri can't be used in other modes like RB.