Using 'condescendingly' can hurt feelings, implying disrespect towards someone's abilities or knowledge. His condescendingly simple explanation made her feel undervalued.
Pro Tip 2/3
Social Context
Considered rude and can insult, use cautiously in social situations to avoid offending others. She felt insulted when he condescendingly said she wouldn't understand the topic.
Pro Tip 3/3
Subtle Use
Sometimes used subtly in sarcasm or humor, but be aware that it may still come across as mean. He condescendingly joked that I was finally catching on, which didn't feel nice at all.