We...ll, there are some that would probably be ok.....mollies, for example, seem to do fine in FW (although they seem prone to Ich). It would depend greatly on the type of fish we are talking about. Some go in between brackish and fresh, some live in brackish their whole lives, some live in brackish then go marine.
The species that tend to stay in brackish areas and those that migrate out to the ocean would, IMO, not be suitable for FW. If I was keeping a fish that I knew was considered a brackish fish, I wouldn't try to keep it in FW.
Fish tend to have longer healthier lives when given the proper conditions.
So your question is kind of broad and not easily answered (at least by me). People argue about it both ways. I am one of those who think "No they wouldn't be ok in a 100% tropical tank", which was what my first reaction was to your question. But I don't want to come off as a know-it-all sounding brackish snob...lol
I am sure Brack-man (and other brackish keepers) will get on this and give their 2 cents as well