“Also, the term “baalei teshuva” includes a very broad range of people.” “Really everyone is a Baal Teshuva as at least once a year around the Jewish New Year everyone repents so defining it maybe difficult.”
I think that in this case (as well as the usual way the term is used), we are talking about people who have done teshuva for things which they hadn’t known were wrong in the first place (people who didn’t grow up Frum).
That is why there is less of a concern that they will revert back to their old ways. They were only doing things they hadn’t known were wrong at the time they did them.
On the other hand, if someone does teshuva for things that they knew were wrong when they did them (like what everyone is supposed to do on Y”K and every day for that matter), there is more reason to believe they will return to their old ways.