Comparing Spellfire and Zhentarim

Both of these feat chains are from the same book, so I wanted to compare them for balance purposes.

Spellfire Spark is an Origin feat.
Magic Absorption.
Spellfire Flame.
It has limited use for Flame. You get a scalable damage cantrip.
Absorption works once per turn. I'm thinking that's when every enemy takes a turn.
Also, depending on how you define "magical effect", it will make a +1-weapon equal to or worse than a non-magical weapon in damage.

Spellfire Adept is a general feat
Fueled Spellfire.
Searing Spellfire..
Fueled Can add extra damage to ALL Radiant damage. EDIT: Spell damage

Zhentarim Ruffian is an Origin feat.
Exploit Opening.
Family First.
You can do more damage, using Hit Dice, on an Opportunity Attack which you can only do once per round and may not happen at all. EDIT: No hit dice.
I like First. Advantage for the party on Initiative is great.

Zhentarim Tactics is a General feat.
Retaliate.
Versatile Merc.
I like Retaliate and Merc. The Opportunity attack has its limitations as stated previously. Also, imagine you're a Fighter with mid to upper 20's in AC.
You would have permanent Expertise in a skill, I think. However, it doesn't say you can change the skill. My guess is you can. EDIT: You can change on a long rest.

There seem to be substantial benefits to using Spellfire over Zhentarim. I would argue even for melee classes. Except for Barbarians.

Couple things to point out here:

Quote from FoxMulder >>

Fueled Can add extra damage to ALL Radiant damage.

It's only radiant damage from spells, not all radiant damage, and it's only once per turn. (That last part is particularly relevant because with Spellfire Spark you can cast Sacred Flame twice in the same turn, but you'd only be able to get this damage boost on one of them.)

The fact that this benefit is only applicable to damage from spells and requires you to spend Hit Point Dice to get it means that it may not as useful on a melee-combat-focused character, since they're unlikely to be using spells as much and may want to save their Hit Point Dice for healing themselves.

Quote from FoxMulder >>

You would have permanent Expertise in a skill, I think. However, it doesn't say you can change the skill. My guess is you can.

It very much does say you can change it on a Long Rest. It would just be a worse version of Skill Expert otherwise.

I was adding stuff without the entire description available in the post. I had to remove stuff from my first try, due to copyright issues. I meant to say that Spark is more likely to be taken on melee characters for the damage. Absorption can be used every time an enemy hits you with a spell or magical effect. Thanks.
Pointing out the imbalance between those two chains was my point.

I think your problem is twofold.  First, not reading abilities you are evaluating - a lot of your questions could be answered by just reading the text.  Second, you are evaluating the items individually and not in the context of their design.  They are not designed to be balanced - they are designed for a specific function and do different things.  You might as well be comparing a bolt to a gear.

Spellfire is… just not great for melee characters.  You are spending valuable feat slots to get an insignificant amount of damage that also burns a valuable bonus action (particularly problematic for Monks and Rogues, but also plenty of other subclasses).  Sure, you also might get an average of 2.5 damage reduction per turn also, but situational damage reduction of such a low value is generally going to become irrelevant fairly quickly.

The second level is useless for most melee characters, other than Paladins or Melee Clerics.  Unless you are consistently dealing radiant damage, you do not get much out of it - the extra damage is too small to really matter, and ignoring resistance is situational and really only necessary if you are so focused on one damage type resistance would cripple you.  That is clearly what the feat line is designed for - those who are going heavily into radiant and want additional sources and ways to punch damage through.

Zhent Ruffian is I have some game design problems with.  It is based around a resource that, other than humans, is DM dependent.  That is just bad game design. That said, the re-roll is powerful on the right character - Zhents attract roguish sorts, and that is clearly what this ability was designed for. Re-rolls are at their strongest when lots of dice are involved, and a rogue should be looking to get opportunity attacks to get extra sneak attacks in per turn.

The second level gives you more ways to get opportunity attacks (and a passive defensive buff against smart monsters that might not target you after they realize it comes at a cost).  It also gives expertise you can change per day to suit your needs, which is fantastic.

All told, I think both feats are really well designed - they are something that can be pretty good with the right character, but are not so generically good as to be auto-includes, and really do not work well outside of builds they are designed for.  Frankly, I think more feats should be designed like these - feats that can make specific characters sing, but will not show up time and time again.