If no one else has noticed, then I'd like to bring attention to a few experinces I've had just to expand our collective knowledge!
Strong boxes have a exponential growth drop rate with how quickly you kill things, for example, if you have a highly augmented ((usually damage and peirce works exceptionally)) CM Jupiter on the end of V.I.P. and activate both spawns for the zomdroids and retreat to the house, when you start to fire upon the zomdroids when they all line up, you will get at the very least 2+ strongboxes that are tier 3 and up, ive even gotten a thulium from this method and 5 strong boxes at a time. Not only that, but I've noticed "natural" drop rates ((non boss drops)) have higher quality gear, especially nantoniums, and strong boxes from collection rewards usually are extremely amped up in value, as in a teir 4 strong boxes at level 40 from collections dropping a red 10**Titan HUD helemt. The only thing i have against my experinces are that on onslaught, this has never happened for me, but it seems on onslaught the enemy A.I. limit is lower than V.I.P., possibily due to it being the first level.
Note: All ive previously stated has been experinced on single player.