My difficulty scale for DYOM missions in general:
11/10 - Unfair mission, almost impossible
10/10 - Difficult mission, many failures until you win
09/10 - Difficult mission, some failures until you win
08/10 - Difficult mission, but possible to pass on the first try
07/10 - Average mission, you will probably pass on the first try
06/10 - Average mission, you usually pass on the first try
05/10 - Average mission, you will pass on the first try if you don't mess up
04/10 - Easy mission, you can pass without paying much attention
03/10 - Easy mission, but there is still something to do
02/10 - Easy mission, you can ignore pretty much everything
01/10 - Easy mission, to get from point A to point B
00/10 - Free mission, you don't need to do anything

My rating scale for DYOM missions in general:
5/5 - Interesting, presents a good balance between story and gameplay or has a great story/gameplay in particular
4/5 - Interesting, but it's too simple or has clear flaws in the story or gameplay
3/5 - Uninteresting, but still features a story or gameplay
2/5 - Uninteresting, has a bad story and bad gameplay
1/5 - Disposable, everything very poorly made or broken