Libra explanation?

[Starving]

Libra is a very interesting item, but I never seem to understand it. When I pick it up usually or accidentally, all my good stats are completely thrown out the window for stats I didn't really need. Or I could pick it up, get good stats, pick up another item and see all my stats go to hell.

How does it work?

[John Orbit]

What it does is balance out your stats. So, if you have really high range, it will drop your range and boost everything else. It also balances your stats every time you pick up a stat-up item.

http://bindingofisaacrebirth.gamepedia.com/Libra

[Rio]

"Stats are capped before Libra's balancing is applied. This means that if speed is maxed out (2.0) already, a Speed Up effect will not increase any stats at all."

This part is important. Some stats cap fairly easily like tears (at 5) or speed (at 2.0), while other stats have a very very high cap such as range. So a tears up pill becomes useless very quickly while a range up pill does not I believe.

So a build with really high range, a higher number tears like 28, poor speed probably benefits more from libra than a fast build with good tear rate. Also luck is really important "Luck upgrades increase all other stats while still gaining the normal amount of luck." since you are never hitting the luck cap (so placebo + luck up pill is really great).