When should you be honest

My husband is struggling right now. Going to therapy and on medication. He has made some decisions that effect our lives. I am supportive but I also am the worrier of the relationship. I worry about the future and not the here and now. He asked me what I was thinking about his decision and I wasn’t completely honest because I know he is in a fragile place and want him to succeed. When is it best to be completely honest about my feelings or just try to keep the smiling face?