Reliability and more
The Montreal was never imported directly into the US market, so all cars here are so-called gray market cars. Parts and service are far harder to get, and the car has several known issues regarding engine reliability (very expensive to repair), and are prone to rust. GTVs also tend to rust, but there are so many more of them you can still find a good one if you look. Montreals are much rarer, so the choices are few. Most Alfa specialists have seen so few, and so rarely, they don't really know the car well. Parts will be a constant search even for the commonplace. If you don't have a support network already established, the car will probably be inoperative for long periods for what should be small problems.
I have never owned one, but my opinion is based on hearing horror stories from several former owners, and knowledgeable technicians.
In any case, if you get serious about the Montreal, be sure to have it thoroughly checked-out by a competent Alfa specialist. That alone will alert you to how little support there is available for these cars, and may help you avoid the financial suicide of buying a bad one.
Good luck!