I can more or less confirm that the more you play the less your chances will be to get anything rare.
I play this game daily for 3+ months, not really wasting any stamina during the daytime, I've maxed out almost all of my SRs (48 including some doubles like Mecha /1st form Friezas, Android 14s etc), almost without using training items.
So I had had a lot of chance to do Multi-Summons, but still I got only 1 SSR, an SS Goku.
Did 1-1 multi during the last two SSGSS "events", and I got this a minute ago (SSGSS Vegeta my a$$):
and the top 10 on this screenshot (during SSGSS Goku):
So I might enter the lousiest SSR account contest having a rank 130 acc...