The new secondary Arcanes are pretty amazing. A bit niche but still amazing.
This is amplified by a current bug with ability/effect stacks losing their damage potential over time despite actually visually increasing the stack numbers (e.g. on Merciless Arcanes). Hopefully this gets fixed soon but for these specific tests this has no impact.
The new arcanes have a lower damage increase per stack but since the stack increases on status effect instead on kill it is much easier to apply the effect. Especially on weapons with high status chance, high fire rate and high enemy difficulty.
The on kill condition is not so easy to achieve on the old Arcanes, especially on high difficulty. The first kill always is the hardest. If you struggle there the Arcane in the long run wont really help you.
The new Arcanes on the other hand do not require a kill to trigger but stat procs which are easy to do.
The downside is the usage limitation.
Cascadia Flare only works on HEAT status procs. If the weapon is not modded for heat damage you won't get any damage stacks.
Cascadia Empowered works on all status effects but only adds a flat (additive) damage bonus... which works great as long the enemy level is not getting too high and/or the enemy does not have cleanse status abilities (like some very few bosses). Efficiency also depends on the type of the status effect applied.
Cascadia Accuracy is great.... IF you are a sniper. 300% crit chance on headshots after rolling. This might work on some builds for sure but I am not that good at aiming actually.
Cascadia Overcharge, 300% crit chance if overshields are active. If you run a warframe which can do that frequently/non-stop then this is a really good choice.... but it's a niche for very few warframes.
The most useful new Arcane here seems to be Cascadia flare since the current meta favors viral/heat builds and with 480% damage increase it is just the highest possible damage buff you can get.
Are old Arcanes useless now?
Merciless and Deadeye are still viable on weapons with low status chance, no heat status, high base damage and/or high crit chance.
Secondary Dexterity is something to use if you are actually don't want to use a secondary weapon but increase your melee combo duration (I really see no other use for that one).
New primary Arcanes suck. Not worth to talk about them.