crazyhead - yes, you have hit the nail on the head. If the man marries the mistress on the rebound - for instance wife has chucked him out after discovering the affair and won't/can't forgive him, then it is in all probability not going to be a very successful relationship. Simply because it is not a great foundation for a successful relationship to have an angry and upset ex-wife and maybe disturbed children as the backdrop to the next marriage.
As a mistress, you would have to be pretty naive to imagine that this would be a good foundation for future happiness. And also astonishingly lacking in empathy for the ex-wife (assuming she wasn't a complete cow).
I can think of one example where this is exactly what happened. The man had an opportunistic affair with a much younger woman who was flattered by his attentions. For him it was an ego-stroke and based on sex. She naively assumed it was far more and they would be together were it not for his wife getting in the way. In any case, it is most definitely a case of 'affairing down' as, had the husband been single, she would not have stood a chance in hell.
There is the inevitable discovery-day, partly because the mistress is far from discreet, wants to come between the husband and wife, and claim the errant husband for herself. Wife's world is turned upside down - not helped by the fact that the mistress is using all the guilt and drama of the situation to drive a bigger wedge between husband and wife.
Errant husband is full of guilt, shame and remorse but it is too late - wife is a woman scorned and feels her pride is at stake (especially with predatory mistress not backing down) so demands a divorce.
Husband, full of guilt and shame, is left with the booby prize - the mistress who had never had any intention of spending the rest of his life with. She was simply a prop for his marriage in which the sex had become mundane and/or infrequent to non-existent. And he hadn't realized that, at heart, she is a bit of a bunny boiler who will not stop at anything to 'get her man' and is a women who sees other women as competition (some men can't spot this).
Mistress is elated at her 'conquest' and manages to get the still shell-shocked husband (who cannot understand why his ex-wife is so angry) to agree to marry. Making the best of a bad job, and figuring that it is better to make at least one of the women in his life happy, he agrees to marry.
The petals have scarcely faded from the second wedding floral displays than husband already has itchy feet. He feels he married under pressure, and beneath him (both true) and he also has not exorcised his demons from his last marriage.
In my opinion, a remarriage can really only work if the couple were truly ill-matched in the first place, or the marriage was abusive or desperately unhappy OR the marriage was okay but the couple have grown apart and can split amicably. If the split is acrimonious and the couple have not learned lessons from what happened then it is almost inevitable that history will repeat itself.