It's older, but there's a video on YouTube that discussed and tested invader treasure ratios versus level of difficulty.

pilou said:

... I just recently unlocked the level II invaders... All I get is stupid shitty things ...This is utterly frustrating. Does anyone why this happens ?

well, it is possible that your attack on lvl II are yet too much weak... the critical hit is the thing who give you better reward that expected... max mat with lvl 1 is usual but you can have unusual with critical hit... lvl 2 is max unusual but once your critical hit increase, you can have rare...

just unlocking a lvl of invader don't make you a expert for this type of invader... you need more research, better gear, etc...

pilou said:

... I just recently unlocked the level II invaders... All I get is stupid shitty things ...This is utterly frustrating. Does anyone why this happens ?

well, it is possible that your attack on lvl II are yet too much weak... the critical hit is the thing who give you better reward that expected... max mat with lvl 1 is usual but you can have unusual with critical hit... lvl 2 is max unusual but once your critical hit increase, you can have rare...

just unlocking a lvl of invader don't make you a expert for this type of invader... you need more research, better gear, etc...

Also your offense is extremely important the higher it is the more energy you can take from the invader, I have got legendary material from a lv5 invader on a number of occasions, You can always get the next level up of material that the invader states you can receive for example you can receive usual material from a lv1 invader even though it states the max material you can get is simple or you can receive rare material from a lv 3 invader etc  but your offense has to be high enough and you also need to get a critical hit too, Also the increased attack is key each time you attack the invader your next attack is increased by 15% you can increase you sustained attack by adding hero skill points too it and by upgrading your invader knowledge for sustained attack I and Sustained attack II. Hope this helps.

Thank you both. I'll try to enhance my knowledges and see what happens :-)