Obviously a relationship with an adult child's partner will be totally different from the one between said AC and his/her parent....and as such will require a lot of level headed thinking. That is so obvious it's hardly worth saying. However, a relationship between a parent and an adult child where one fears being honest with the other is, in my opinion, a relationship with something missing.
As for the partners, if someone is determined to cause a rift, any and everything will be grist to their mill. My daughter-in-law cut me off because, travelling through her country, I commented that it was beautiful, but I personally missed our "green and pleasant land".