EzekielRaiden
Follower of the Way
A 50/50 mechanic is (essentially) guaranteed to wear off after at most four failed saves (chance of failing 5 consecutive saves is less than 5%, in fact exactly 3.125%.) When spellcasters can target weak saves and increase their own saving throw DC (through stats or through items), you can easily get that to be a 70% chance to fail. A weak save may be at +0 or even -1, while a level 4 caster with a single magic item can easily have a DC of 8+4+4+1 = 17, possibly higher; that would mean needing to roll at least 17 (possibly 18), which is an 80% chance to fail. Even if we don't include the magic item and presume a +0 bonus on a monster's weak saves, you're looking at a 75% chance to fail. The equivalent probability (less than 5% chance to fail that much) is twelve consecutive saves, meaning ~11 rounds of maximum effect as opposed to only 4.Save each round to end the effect is not wildly different from 50/50 each round to end the effect. Different yes, wildly different no.
And if we consider average effect it's even worse: 50/50 gives exactly 1 round of expected total value. 75/25 gives exactly 3.
I'm pretty sure 3x effect--particularly in a game where "three rounds" may as well be "until the end of the encounter"--is rather dramatically different.