even the best rare champs aren't really on par with their epic or lego counterparts. it sounds like you're going off ayumi's list, so let's look at those.
we'll start with warmaiden. she certainly is a good champion and given how useful AoE def down is, she can find a place in every team. she's also easy for all players to get and book since she's farmable. beyond that though, she really brings nothing to the table. her other abilities aren't anything special and her stats are fairly mediocre (a common problem for rare and below champions. their base stats just aren't high enough to be competitive in the long term). typically a player will only use her until they get a better def down champion and you already have 2.
athel and apothecary are similar. athel's weaken debuff can be helpful and her 3 hit a1 is great against fire knight, but beyond that she's just a damage source and one who is easily outpaced by better champions. apothecary is a little better due to the utility he brings to your team, but there's still nothing that can't be easily replaced by other champions.
coldheart is a bit of an exception to this rule and is argueably the only rare champ in the game to remain viable even into lategame (outside of specialized areas like secret rooms). this is mainly due to there being fewer than 10 champions in the game whose damage scales based on enemy max hp, making her a3 much more difficult to replace. even then though, she's a bit of a seesaw champ—great in some areas, terrible in others—whereas most S rank epic or legendary champs will be strong everywhere.
regarding the champions you specifically mentioned, both have their uses but aren't the best investment of resources for you at the moment. kurzad offers an increase accuracy buff, which is also quite rare, but nothing else in his kit is particularly useful and increase accuracy isn't one that's likely to be essential all that often. soulbond is honestly a pretty good champion, but i can see a number of champions in your collection who you'll end up getting far more use out of in the long term, so she's not really a priority at the moment. i would consider gorgorab, uugo, delinia, apothecary, and fodbar much more important than soulbond and by the time you build them all, you'll likely have gotten other champions who are also more important than her. you might end up bringing her to rank 6 eventually, but it likely won't be for a long time, when you're building champions for doom tower or faction wars specifically.
to answer your other question, no, kael isn't really the best nuker. the reason he hits harder than your other champs is partly because he's your only rank 6, which is a world of difference from rank 5, and partly because you have zero good damage dealers at the moment.