For my Zane I do the same as sunrise, for Fl4k I like to rock cryo and rad combos especiallyon mixed health enemies and Amara can have it all but I usually stick to the main three with he because she gets great synergy, but rad Amara with the ricochet bullets and shared elements makes her over the top crowd control.
Against weak/medium enemies switching weapons or modes may be undesirable, because it’s annoying to switch all the time and it actually costs DPS. A “bonus damage after action skill end” anointment on a gun may be another good reason not to switch weapons (because it’s lost if you do).
Radiation is very good at doing away with flesh+shield mobs without switching, and Cryo with flesh/armour mobs, much more so than physical or any other element.
Of course cryo has other uses, like slowing down enemies that are charging at you.
Brawl Amara actually goes really well with cryo because a frozen enemy suffers 3x melee damage. High efficiency cryo shotgun + Blitz + a lot of melee bonuses = yum.
I run a radiation Zane, it’s great for groups, I have a decent focus on AOE and the explosions keep the damage spreading. As others have said, it’s weakness is armor so I keep a Maggie to help with that but every other item is radiation focus. It’s not as good for focussed damage though, he definitely struggles more on bosses compared with my Flak or Amara though. Also if you’re working radiation into your loadout, be aware that when you kill an enemy and you get the radiation explosion it can damage you too, I use red suit so I’m immune but unless you’re going full radiation build it’s definitely not the best shield due to its low capacity.
Mostly there general use elements but preform better with the special relics that match there elemental type. There are some Relics like Atom Bombs, Cryo Stones, and so on that increase the effect of these elements.
Honestly it’s better to look at where they are not good.
Cryo is not very effective against Anointed Bandits who can be slowed by not Frozen. It is also not as useful on bosses whom for the most part can’t even be slowed. It dose find use in helping you manage add enemy spawns in said boss fights.
Radiation is pretty amazing across the whole game but its versatility is also why at times it will not appear to be doing a huge amount of damage unless your building around it or using an exceptional radiation weapon. Only Iradiated and I think Puke Scags are immune.
If you do enough damage for your difficulty level use Radiation or Cryo because they have the most utility while being the most neutral of elements. Radiation and Cryo are also always gonna be useful as ASE anointments (the best imo) because of their utility.
If you do not do enough damage or the enemy is heavily shielded/armored use shock/corrosive. CoV typically aren’t that heavily shielded but Maliwan are, especially bosses and nogs. Most things aren’t super heavily armored outside of Hardened Badasses/Jabbers and certain bosses, but armored enemies are super annoying so you’ll want to kill them fast anyways.