Maybe I'm lucky but I've never been fired for telling the truth and the truth is easier to align to. "There's a bug in a third party library that's part of the critical path "We can make it harder to hit the bug but can't fix it until the vendor fixes it", or "user growth has revealed performance issues no one thought we'd encounter this soon, we can spend three times as much money on infrastructure to mitigate it for the 2 months it will take is to fix it or lose customers due to performance" or "our biggest customer didn't know what they wanted until we delivered the first iteration, now they want something that isn't at all what we thought we'd be building. We can build it and be profitable or follow our dreams and die".
Again maybe I've been lucky but honesty has worked well for me.
People that surround themselves with “yes” people often fail to digest the truth. So as others have mentioned, they will “take it under advisement” and then you will slowly get phased out or put into “busy work”.
Second option is to hold your tongue, watch them struggle, and eventually fail over a long period of time. Usually 1 year but have seen it fold in less than 2-3 months and the leadership effectively canned the next quarter.
This is not really about honesty - if someone asks your opinion you can explain your concerns diplomatically - but if they don't then you can also keep quiet.
It's more about whether you're going to actively point out a problem with a plan that someone higher up than you is trying to take credit for. i.e. they will feel personally attacked - that their judgement is questioned - when you speak up.
It's an extremely difficult thing to do without making enemies and enemies last a long time and 1 enemy is more damaging than can be made up for by lots of friends.
Again maybe I've been lucky but honesty has worked well for me.