They definitely will need to get hold of your ex-partner. It doesn't need to scupper you, they will take into account if he has mental health difficulties, and tehy have certainly met there fair share of abusive and malicious ex partners if that is the case with you. Plenty of poeple with malcious ex's still get approved to adopt!
The assessment may or may not be traumatic depending on the person. You will have to go over your pregnancy attmenpts and miscarriages and your feelings in depth howeve. You will also have to go through your past relationships in detail, and of course your current one!
There is a reason it is bad, though, honest. The children who, as you rightly say, need loving nurturing homes, are often very traumatised, abused children. They are experts in survival at all costs, survival of the fittest, and they can and will test your relationship to its limits. You need to be rock solid together, as a young child can still expertly find any cracks in your relationship, and drive a wedge in there, and try to split you to their own advantage. it sounds kind of crazy at first, but it is how they automatically operate - they know for a fact that they can survive in chaos, so they try to recreate that chaos wherever they go, and they find a loving nurturing home a threat to them. So that is why they will go over all of your relationships past and present in such detail - what coping mechanisms do you have, where are the cracks if any, what caused other break ups in the past?? That's also why you need to have been together for a time - you will need to be this solid team together.
So for me it isn't the process personally, but afterwards! If what i read is correct, then a third of adopted kids have some problems, a third have moderate problems, a third have severe problems.
Some can mean lots of things but generally managable behavior - maybe they get very over anxious and clingy, or never go to sleep, or can be aggressive sometimes or have ADHD or sensory issues, lots of delays, or big trouble making friends etc... probably a combination of lots of these
Moderate - you are talking my DD's in this category from my perspective. Both of them are emotioanlly imature for their age. DD1, she has attachment issues, can be controlling still, still vulnerable at 24.. but when she came she was constantly attempting to touch me or worse in a very innapropraite way, she coud have 3 hour screaming tantrums, she could not manage mainstream school at all, she tried to control a lot of things and attempted to be the boss most of the time.. this was all day every day for several years. She could not sleep at night much, needed the security of door alarm on her bedroom door, needed my total 24/7 supervision around all other adults and children becasue otherwise she wold try innapropriate things with them etc etc.
Severe - Now you are talking kids with severe RAD (if you wan to adopt, you need to know about this), multiple mental health problems, school exclusions, police involvements, violence frequently and most need to move away from family into treatment centres for wrap around therapy.. many of these placements disrupt altogether.
So yes, adoption is many things, a lot of them positive, just look at my amazing kids 
. I don't know any adoptive families who do not love theri children from the bottom of their hearts whatever their problems but it is HARD and NOT for the faint hearted.