i agree with you on this and thats why i had used words “chances are rare”. Arrange marriage is a gamble in total and you never know it all until you actually start living with that person. Most of the times pre-checks done by parents work well but it goes in a wrong way too in some cases.
When i mentioned the importance of family background, trust me it works a lot. Though there is no guarantee and any one call fall on the wrong side but if a guy or girl is coming from a strong family background, he/she would think 100 times before taking any odd step.