None of these issues I could have foreseen. If any of you mystic megs out there would like to give me lottery ticket numbers that would be grand.

People who have never tried step-parenting have no idea of the multi-faceted issues you end up dealing with. Try reading through the posts on this forum and imagining if it were you having to cope with this kind of criticism, day in, day out, in real life.
When parents split, the way they parent their children can change, even without the introduction of new partners. Without two hands to the pump, standards can slip and indulgences be made out of both guilt and exhaustion. As the years go by, those children can turn into mini-mates and mini-wives.
A few years on, now add a new partner to the mix. The new partner is not only the family outsider from the start, but the children have become used to all sorts of allowances and excuses being made for any bad behaviour. They are also used to constantly being granted adult privileges. Well, maybe if you are a teacher, you get to bring classroom discipline with you for both those kids and your new partner - or maybe not!
Unless the parent is prepared to make significant adjustments to that parenting style, living together as a blended family is going to be utter misery for any new partner. Many of the problems on this forum come about because the parent and their wider family are not prepared to adapt, and expect to simply bolt an additional person onto their existing family as if they were a spare part for the washing machine.