A procedural fairness letter gives you a warning, so you should work to answer the questions raised in the letter to resolve these issues prior to refusal. If your application is refused, you will receive a decision letter from the IRCC, the letter will detail the reasons as to why your application was refused.
If in fact you were in a common-law relationship (even if you didn't realize that you were), it is still a big problem.
It is very simple. If you and your husband ever cohabited together (anywhere in the world, not just in Canada) for one year or more at any time, before he applied for his PR (or even during the processing of his application) and the relationship was ongoing, you were common-law. If it is determined that he was not truthful in his PR application, he could lose his PR and could never sponsor you under the spousal sponsorship process.
As great as these forums are, sometimes it's best to confer with a qualified lawyer that can give you solid answers/advice, as long as you both give him/her/them solid truthful answers about this matter.
Good luck!