mRNA vaccines are completely safe - Obviously false just like it is false for any vaccine since there are always going to be rare side effects in some members of the population. They very well may be just as safe as any other vaccine, but the truth is there are some things we won't know until a lot more time goes by. You can't say what the long term side effects are of something until it has been around for a long time.
Wind power is better for the environment - Similar to above, we don't really have data on the full lifecycle environmental costs of large scale wind farms. This is especially true because a good portion of the impact depends on how long the turbines last, what type of repair/recycle technologies are developed, and what type of policies get applied to recycling them. We see some things that look promising but others that are concerning.
Saying "mRNA vaccines are completely safe" is obviously not pedantically true. The same could be said of anything, not just mRNA vaccines. Nothing is ever absolutely 100 % safe.
It would, at least in theory, be better to realistically say what we knew and didn't know about the risks.
But public communication about those kinds of risks and tradeoffs is hard. If you say there might be any kind of a risk at all, some people are going to get hung up on that or grossly overestimate those risks compared to the benefits (or to the risks of not getting the vaccine -- also not something everyone would be affected by but clearly a non-zero risk).
Taking that into account, "completely safe" might be a better approximation than most other ones you could make.
While I understand what you are saying, I would disagree. Saying that something is completely safe when it isn't (as you say nothing is completely safe) makes it appear that people are being lied to when someone does have some type of reaction.
I think we are better of thinking of risk in terms of every day risk management. Comparing risks to things like driving 10 miles, getting hit by a meteor while sleeping, etc. can help create a better understanding of the risks.
I'm not sure I agree with it either. I'd personally much rather take a realistic estimate (in cases we have one anyway) than a simplified half-truth. But I think I can see a rationale for why some people doing the communicating may opt for the latter.
Comparing to something else that people might have a more realistic intuition of sounds like a good idea.
Wind power is better for the environment - Similar to above, we don't really have data on the full lifecycle environmental costs of large scale wind farms. This is especially true because a good portion of the impact depends on how long the turbines last, what type of repair/recycle technologies are developed, and what type of policies get applied to recycling them. We see some things that look promising but others that are concerning.