FCPx saves every time you do anything. And it will do so UNLESS you run into the infamous "Undo" BUG.
What the undo but does, is to quit "recording" any of your steps. If you make quite a few and dont know that undo has stopped working (becuase you did not need to undo) and then quit FCPx... FCPx when relaunched will open the project at exactly the state where undo stopped working.
Quite a dangerous bug indeed.
Here is a workaround and steps to consider... At least I CONSIDER thoese steps every time b4 quitting
1) Click the edit Menu, If undo and redo are both grayed out proceed to step two
2) Duplicate your current project (if you have been working on several duplicate those too) (Use Project Only Option)
This will ensure that hours of work have not gone lost.
I have reported this bug back in 10.0.4 but it has yet to be fixed. So perhaps if all those 16,000 reading this thread would use the feedback option in FCPx.... Such NASTY bugs would be fixed much faster.
Some folks have the tought luck of running into this bug - some dont. But is IS there !