Long story short, it's completely over-engineered by a bunch of intellectual engineers with no focus, no discipline, and no oversight. It ended up not delivering on any promises it made, and there were a lot of them.
I was warned left and right before presentations and meetings, "this customer hates your product because of ...." I started off every meeting with saying, "we're rearchitecting the product, this is how we're doing it, this is the tech we are using." Immediately there was a sense of relief from customers, followed by questions like, "why can't <current product> deliver <feature> that was promised?" I'm completely honest with bad decisions that were made and how it impacted the feature. Sure, there is skepticism on what we are doing, and I tell them they should absolutely be skeptical based on our track record. The result has been customers who have hated my product now offering to work with us on development.
I've also been completely forthcoming on configuration, security, resources, and setup issues I am finding, many of them are absolutely freakin' insane. I've flat out told customers it's frankly embarrassing and never let us do something like this in the future. The best feedback on this was, "At least you're telling us something. We usually get silence from this team."
God, this is the most depressing job ever.
First off, it helps I've been 15+ years as an engineer, 5 as an engineering manager, and throughout have the community contributions in the field on my resume. I instantly spotted the problems when I was given an architecture diagram on day 1 and discussed what I would do differently. All that gives credibility.
If it's internal audience, I am brutally honest. The organization needs to know this wasn't happenstance and bad luck that put us where we are now. It was a deliberate series of bad decisions based on a poor engineering and product culture. Now, for better or for worse, we are tasked with paying the debt.
There's a certain class of customers that are sister companies under the same parent. I'm honest with them, too, but go on the offensive. They have abused my team and our company in the past, and unfortunately, we have let them. I am more than happy to fire back and go toe to toe with bad behavior, and at the same time working to fix critical support issues.
For external customers, I've had remarkably good response in listening to their complaints. I am honest in discussing, in deep engineering detail, how the new product will address their problems, where issues might still be, and development timeline. I like to think the credibility portion comes into play here. In the past, customers were just told, "We'll look at it" and "We'll fix it" but nothing was ever planned.