I think it's important that when you're making a speech or presentation that you say what you say with conviction and sincerity. When we're talking about objective criteria, that is that which can be proven, we need to know our material.
That said, be aware that you can put a subjective spin on any objective 'fact'.
When it come to subjective matters, your opinion based on your knowledge and experience should be valued. However, not everybody is going to agree with you.
Look at the divisions caused by the UK EU referendum campaign and the US elections 2016. Both almost 50-50 in the ballots. That's a lot of disagreement and conflict. A cause of bitter division even in the closest families.
When I talk in class I draw extensively from my depth of research and my practical experience in the field. That's where my strength lies, but not everybody gets that (at first).
That's fine too because our experience of life is completely subjective and agreeing this upfront means that we can disagree and yet respect each other's opinion.
If we disagree, I'm not afraid of asking them if they have something better or more effective than what I'm offering. I'm curious to learn if they have. I am willing to be flexible in my thoughts and opinions. If somebody comes up with a more effective tip, tool or technique I will be the first to adopt it. That's how we learn and grow: by keeping our eyes on the horizon and seeing what's out there.
It doesn't matter what you say or how well you say it, not everybody is going to buy into it. Don't take it personally. It's the way life is.