Context is really important. If you want to go down that road of possible ambiguity, then "one and a half million" could also be interpreted as
1 + 500,000, right?
In reality, the context will be such that nobody will be confused no matter which of the two you use. How often does the number 1,000,000.5 come up? Extremely rarely. 1.5 million, on the other hand, comes up all the time. In conversation, I think you'll hear both of these phrasings used for 1.5 million with very little confusion, because everyone will assume you mean 1.5 million. So, in that sense, both can be considered acceptable. (Compare this to saying "fifteen thousand hundred", which you'd have a hard time calling "acceptable".)
However, in a publication of some kind, it is probably seen as better, clearer writing to use "one and a half million". In this kind of one-way communication, you only have one chance to get your message across, so you should make sure it is understood. If I were told by my editor to only write "one and a half million", I would consider that a reasonable edit.