One has to be concerned about these things yes. I believe it's OK to state a negative personal opinion, but if you post wrongful information on the board (stated as fact) that could impact on a project or a person's career, then you could be at risk for liable.
An example would be "I've heard that condo project "X" used substandard plumbing and because of this, their toilets are all backing up"
A post like this could seriously damage the reputation of the builder and the building owner and could impact on the ability of the owner to sell units in the building. If untrue or simply hearsay, then yes, I think you could be placing yourself at risk for litigation.
On the other hand, if you simply stated that the same condo project had hideous yellow vinyl siding that you detested, than that is just a personal opinion and no one is going to sue you over that........