I'm a relative newbie, so I know what you mean about being out of your depth. I found that pretty much everyone expects that and will tell you not to worry. I work in a difficult and highly technical area of advisory, but I've been told "don't worry about it, we cover such a broad area you could be here 15 years and not be an expert on anything"! I think because the civil service encourages people to move around, there's no expectation of knowledge and there's an expectation that you'll spend your first few weeks "reading in". I've been hitting the textbooks hard for the first time since uni. I still haven't found my groove exactly as I'm very new and still need a lot of supervision, but I'm a lot more settled now than I was in the first month where I wondered if I'd last six weeks!!
In advisory you meet your policy clients, or they email a query, and you have to do the research and put together an answer that sounds reasonable and is a decent argument. Some areas are easier than others - a colleague who worked in education said you could become an expert on the Education Act relatively quickly, whereas with DExEU it's obviously all new ground to break. Litigation is more of a constant workflow than advisory which can be a lull during the purdah period for example.
Pros are obviously the hours, as everyone acknowledges family situations and working from home is encouraged. Colleagues are friendly and welcoming. There are some fantastically brilliant minds doing very intricate work. It's great if you want to have an inside view of how it all fits together. There's no pressure to dress or appear particularly corporate as long as you are neat, tidy and not flashy. Many younger staff do wear suits a lot, but lots don't.
Cons: the money if you've come from a big private practice (I haven't, so it's fine for me). The sensation of feeling out of depth is a bit overwhelming at times too, but when you can put into advice something you know is right even though you had never heard of the topic two days ago, it's a good feeling. Satisfying.