I honestly just started using Magnetic on almost everything. I found I wasn’t needing to minmax mod compression, which is what Toxin does by saving you a mod slot. Meanwhile mag/corrosive/heat gives you universal coverage against basically all health types outside of boss fights (where you want Rad, usually, or Blast for Wolf farming). Once the build is done it’s done, no swapping for different factions at all.

Magnetic also has a pretty desirable interaction with Galvanized damage-per-status mods, tacking on an extra element.

Off the top of my head, the big exceptions I personally made were Hek, where I wanted Rad for more boss damage, and Chakkhurr, where I did actually want the extra mod slot for more crit chance.

Melee is a different matter, and where I agree with either more Tox, or potentially pure innate Heat if you want to lock the weapon in as a Heat Inherit endpoint. Tenet Agendus will want Electric though so you’re not stuck with Corrosive.