Houses are not merely investment vehicles. They fulfil emotional needs and for many people contribute heavily to their personal identity. Many people consider their homes to be their most prized possession and an indication of both their financial acumen and their social status.
As an appraiser I run into this all the time. When you tell a homeowner that their home isn’t worth what they think it’s worth you’re not just commenting on the price of the home. You’re also saying that the homeowner isn’t worth as much as they think they are; that they aren’t as smart as they think they are; and that they don’t have as much social standing as they think they have.
Truly, it’s probably better for your relationships if you just keep your opinion on this to yourself unless cornered.