Doiron and Morena,
My family is in very similar circumstances to yours so I thought I would share my experience.
I am a Canadian-US dual citizen who resided in the US for over 15 years. My husband is a US citizen and our 4 children, all born in the US are dual. We decided we wanted to move to Canada but for the last 2 years my husband has been a stay at home dad and I have been working full time. We don't own any property in the US and have been renting for the last several years. We lost everything when my husband's business failed a few years ago so we really have no financial assets of any kind and I was worried about the lack of ties to the US as well.
We all came to Canada together in March and like you both, we were a little concerned about the border crossing, so we left most of our belongings in storage in the US so we didn't throw up any red flags. The officer at the border where we crossed with both of our vehicles and 1 small u-haul trailer was very helpful once we explained our situation. He gave my husband a 6 month visitor visa in his passport and said he should be able to renew it if the PR is still not processed when it expires. He said that since we are married and have been living together, we can't really separate our possessions into his and mine so he just let me bring everything we had with us listed in my name as a returning Canadian.
We were up front with him that we understand the law and that if for any reason my husband is not approved, we will all just move back to the US as we cannot be separated. He didn't seem concerned and now I sort of wish we had just brought all of our stuff then since now we probably have to wait until he lands to get it. We have been in temporary furnished housing for the last 2 1/2 months but in a week we will move into a completely empty apartment..... we are thinking of it as camping
The one thing that was a concern was that we were going to keep one of our vehicles registered in the US until my husband got his PR but the border officer at the Canadian border asked us if we had a physical residence in the US and since we don't (we just have a UPS store address for mail), he said we were not allowed to keep a vehicle registered there. So, we had to import both of our vehicles, which was a hassle because we hadn't done the advance paperwork and export from the U.S. of the second vehicle beforehand.
I'm sure the fact that we have been married for 14 years and have 4 children together proves the legitimacy of the relationship but I was still concerned about them not allowing my husband to accompany us due to horror stories I have read. In our case, it worked out fine, though I must admit I am still nervous about crossing the border again before he gets his PR even though I am dying to cross to do some shopping. I still worry that a different border officer might refuse my husband back for some reason.