It's not all doom and gloom, but you have to consider carefully the needs of your children. It isn't like having another child- it's far more intense. You don't necessarily get the information you need ahead of time. Sometimes the information simply isn't known. The case I referred to, it was the child's first placement so they didn't know to expect those behaviours. The issue was their reluctance to move him because, essentially, there was nowhere to move him to.
We started when my youngest was 7, and always took children younger than ours. It worked well for us, for about 10 years. I burnt out, however, as the level of need began to outstrip my energy and emotional resilience.
You are dealing with big, big issues- secondary trauma where you take on/reflect trauma the child has experienced; loss when children move on; abuse from birth parents; constant observation including unannounced visits from social workers; meetings; health care, school etc.
It's all the mundane stuff plus regular meetings, record keeping. Add in extra attention and all the emotional energy that comes with supporting your DC and FC through traumatic events, past events, incidents and accidents.