Sadly no clue. Every account works different in some ways. At start I was unlucky getting nothing ending with mix mechs because can't make a full flesh and adapted to play in that way so I like mix mechs.
Then after 4 months around 5th month the game started to give me lots of legends in fortune boxes like monkeys, platings, claws etc then suddenly stopped and no even farming for fortune boxes used to get them just regular mix boxes reduced because started to get no boxes like the game started to readjust itself in the compensation.
Premium packs are terrible in my case. Just epics and very rare legend and just meat.
Bonker, magma, seraph, heron, terrorblade as example I got them in the forum last summer with Sarah forum offers and still not in use except the heron and terror.
Others items has been purchased with odlffers. I have bad luck for sure.
Jiyoon is one of the players better say free player getting all the time good stuff between few others.
I only can advice, careful with the or will lose your pants. The system compensate sometimes.
Another example it is in OD FARM. If you see back in my progress you will see the heat mech and 2 standard phys huggers incomplete.
The game was giving me for long no even 1 epic. I mean, none zero nada. One day I had 920 mix boxes accumulated. I started to make epics to L30 to build myth meats making legends with them and no epics and just common parts not even rare parts and when I was around 100 the game gave to me power kits, 2 legend max out, 2 legend L15, 2 legend L10, 2 legend L5 and then 4 epics L30 all power kits and use it to max those phys mechs.
Crazy for sure but the game keeps a balance in some way at list in my accounts.
I haven't gotten meta parts generally speaking like those blades, valiant, magma etc, you know the real good stuff. I think I will never see them in my accounts unless a sale someday down the line.
In mean time I play my game and my eyes popping out and saliva dropping when I see the other players getting the good stuff in the free mode.