Is it very difficult to find work after you have been pushed out? Do most people have to transition to in-house or something similar afterwards?
This would tend to vary a lot based on how the economy is doing, what your practice area is, and the ranking of the firm you start out at. Generally, though, it isn't that difficult to find work in the timelines that large law firms give you.
I'd also note that most people don't "have to" transition to in-house, but rather, choose to transition to in-house. If you are looking at a move after, say, 3-6 years at a biglaw firm, it will be easier to lateral to another biglaw firm than it will be to go in-house. More commonly, people actively seek out in-house options in order to have a more stable career (no up or out, which usually means out) and to have a better work/life balance (no longer always-on-call). I have not had friends who are forced into in-house from large law firms, but have many friends who chose in-house for the above reasons, as well as choosing in-house based on geography.