"But" and "however" are used slightly differently.
"However" is used to introduce a statement that contrasts or contradicts what you have previously said.
The sport is most popular with men, however, last year more women took part.
In this example, the second statement seems to contradict, and possibly even replace the first statement. It may be that the first statement has been true in the past but is no longer true.
"But" is used, not so much to introduce a contradiction, but an exception.
The sport is popular with women but only those over the age of 30.
The second statement does not undo or replace the first - it just adds another condition to it.
In your example, I would say that "but" is most appropriate:
The traditional models exist but they may be inappropriate to deal with complex situations.
This is because the fact the models are inappropriate to deal with complex situations does not contradict the fact that the models exist.