"Game of soft wording" is, indeed, doing it's job, but it works only for juniors. So, if you know that a person is learning and you want to keep this person motivated and help you building your product, then playing words here is important.
On the other hand, if you may replace a person to someone who is not so sensitive, then you should do it. If a person does not accepts critique then this person will never become a professional and there is no need in investing your time and money into this person. The person who accepts all the critics even in very rude form will learn much faster and also will be a more reliable partner.
I know many professionals and myself don't like when anyone wastes his time trying to tell me beautiful words my mind wants to hear and not the real facts. With my experience I instantly see when someone plays a word game and lies right into my face. Any real entrepreneur is also talking simply what he wants to say, no more, no less, no playing any Shakespeare's plays.
If someone did a crappy job this is exactly what you have to say without any emotions and never related to personality itself. However, you should obligatory provide strong arguments and facts. Professional should distinguish professional life and personal one. The most of young people, of course, will react and hate you at the beginning because of their psychological self-defense mechanism, this will be forgotten very soon. Be strong and keep going.
Talking about the UI engineer and a design. It is not UI engineer's responsibility to do a designer's job. In that situation best approach is to solve that problem with a designer. UI engineer will accept arguments and work from a good designer but, badly, will accept manager's critique. It's a common problem in a professional life, even if you are a very good designer yourself but you never had an "Art director" title in your life, then all the people around you and who don't know you very well in most cases will just ignore your "design advices" since they will think you are not a professional in that field. That's why you need someone who will have a reputation in the eyes of that person and will be able to support your opinion.
Changes without crushing are impossible. If something needs to be totally rewritten, then it must be accepted. People also learn much faster when they understand their mistakes and work they did was useless so in the future they wouldn't waste their time again.
P.S. Being strong and constructive critic still doesn't mean you have to be an asshole. Never allow yourself to put yourself higher then anyone else, never think you are smarter and always respect the opinions, help, rights and the interests of other person, still you have to think what you say. Respect is what you always must show.
Small personal advice at the end - always remind every team member that there are no "one" or "I" in the team and bad work of a one team member means a bad work for a whole team and everyone is guilty.