Hi and welcome 
Unfortunately, I've known plenty of cases where the parents didn't know either the full extent of the childs history or current issues, or the extent of some of the relevent background information.
Sometimes it's because it genuinely wasn't known. Abuse is often difficult to detect, especially sexual abuse, but physical and psychological abuse can also quite easily go unnoticed and the child be removed for something else entirely. If the child then doesn't talk about it prior to adoption, no one will tell the parents anything. In addition to that, sometimes certain health and medical issues within the birth family aren't diagnosed until after adoption, or are never discovered or mentionned to social services.
Also, there is often a LOT of information on the child and birth family held by social services. If they have stacks of information, you'll never know all of it. They should tell you everything relevent, but sometimes the process of condensing down the information into a CPR and other reports ends up leaving out things that actually parents ought to be told. It also doesn't help that relevent information might be held by more than one agency, and then you have the poor communication problem.
Then, as Devora said, general ineptness results in information not being told.
Also when it comes to medical/developmental/ or really any issues the child has, it's important to remember that SW's are not medical professionals and they can't give you the best predictions as to how things might develop in the future. I know parents who've been told certain things about a childs medical condition, but when researching themselves, found out that actually the prognosis was rather different than what they'd been told. I suspect that wasn't deliberate on the SWs part, but they just aren't qualfiied to give accurate information about that.
And of course, then there's deliberately not telling parents certain information. I have personal experience of this, and I can think of others too. I think it's a valid worry. I also think that attitudes today are more 'open' than they were before, and that there's been a general trend to sharing more information and recognising the importance of doing that as the years have gone on. I can think of quite a few more examples of people being misled between say, 1995-2005 than I can 2005-now. In my case (in the 90's) I am sure the reason so much information was witheld from me was because they thought I wouldn't adopt DD if I was told the truth (to make it worse, she had already been through a disrupted adoption and I would bet my house that her previous parents were also lied to). I also think they wanted to paint a better picture of themselves - the truth was that my DD was failed by them, failed horribly, and so it served their interests to not tell me that.
It depends on the particular council. Some are very good, some aren't so good. Either way, deliberate lying is less common than the other reasons things don't get disclosed, although I think it probably still happens.
Although I had that experience with my first child, I did go on to adopt again (and again). But I went in more aware that I probably would never know as much as I would like or probably should know, for all these reasons.
Adoption is ultimately a big leap of faith. I had to be mentally prepared, and we all have to be mentally prepared, to committ to our child with all we have, knowing that we might well not know everything we should know and that we have no idea how our children will develop or what issues they will have as they get older. I think when you reach that point where you think 'yes, I can do that' then you're ready to adopt. But it's okay to have worries. Everyone has worries during the process, and it's very natural, given what a big thing it is to become a parent (again).
However the more you get involved in adoption, the more you can find out about what information you could get about your child. For instance, the childs foster carers probably know more than anyone about the childs current issues, so you research what questions would be good to ask them. Find out where you can research certain conditions/background issues, and so on. Basically, the more informed you are, the more you'll be likely to find out. We can't guaruntee we'll be told everything, but we can be proactive about informing ourselves!