Lifegiver and Elemental fusions are pretty great. They give rare abilities and often very useful resistances. The simple addition of Foo Dart is incredible at turning turns and SP into damage! Unless meleedudes have Pounce or something they have to spend like 4 turns doing nothing but closing the distance, while rangers get to spend and regenerate SP throwing stuff. And of course, making a Fiery/Charged/Frigid frontliner lets you engage with nasty elemental packs that might otherwise obliterate your demon before it even got close, and you suffer only a relatively minor debuff statwise.
Dark/Light/Mind/Body have less useful resistances (apart from obvious outliers like Turdak's Sanctum) but potentially very nice abilities, no doubt about to get much nicer with the Light/Dark revamp.
Cursed/Blessed fusions are pretty meh. Mainly because if you have something that gives the Blessed/Cursed modifier, it probably already has one or two very usable Bless/Curse abilities, and even if you aren't satisfied with what it has, you probably don't want to bother spending 2000+ currency just to gamble against getting a Redcap stapled to a Freybug for your trouble. While other fusions can potentially give good stuff before you would normally find it, you can get most buffs and debuffs quite early in the game. 2-3 before Anomaly, 3 in Anomaly, and the last two, Protect and Weaken, are found around Turdak, assuming you don't find any wild modifieds. Basically everything is available in the first 1/4 of the tower floors, and the rest is found before or immediately after Turdak. I'm not including stuff like Prepare and Fade but they exist even earlier.
(As an aside I do think the nigh-guaranteed Might from Satyrs is bordering on being an 'Optimal' choice for every game and I sorta think they should be nerfed down to Fervor...)
The mass ones like Flustering Cry give turn efficiency, but the mass debuffs need to be on a frontliner to get in range, whereas the single-target ranged ones generally give plenty of time to be applied if the demon sits smugly in back waiting for the enemy to come to them.
(As another aside I think Rousing Cry and Menacing Shout should be free actions, it's frankly just sort of sad to fight enemy packs that all shout/cry in the same turn and so end up accomplishing nothing).
Then there's Violent/Deadly/Brutal (and to a lesser extent, Lethal/Frenzied/Hulking). For the hefty cost of fusion, you can upgrade a melee demon to have a slightly improved health and damage, and to probably get some redundant attack abilities of which it can only use one per turn anyway. And there are plenty of excellent physical attack abilities early on,(and in every single part of the game), so it's not like early Ice, which has basically nothing going for it but a fish and a watery tart until Bmola or modifieds show up. And while even a Frigid Slime has the potential to do work as a cheap knockoff Asrai or at least offer Ice support for other Ice demons, a Violent Echeneis is just sort of a joke. I like seeing weird stuff like that in the wild and it may well offer some nice skills for me to train or even a nonconventional demon to be my buddy for a while(I still remember a lovely Frigid Zar I had), but the odds of me actually choosing to fuse such a weird beast myself is practically nil, because currency is precious, fusion is expensive, and resists are godly.
IN CONCLUSION, I think it should be considered giving the appropriate physical resistance to Violent/Deadly/Brutal fusions, and maybe even Immunity for Frenzied/Lethal/Hulking. The Frenzied/Lethal/Hulking abilities are actually quite awesome, but still pale a little in comparison to their 2nd-tier magical counterparts I think. Plus the terror of meeting Impact-Immune Hulks when you've been Mighty Roaring everything to death sounds *fun.*
While Blessed and Cursed are a little lame, SOMETHING has to be the runt of the litter. One possibility to help Cursed would be to add the 'elemental' line of debuffs like Ignite, Guilt, Hex, to the Cursed movelist, or perhaps even the melee fixings like Accursed, Evadebreaker,(though that's stepping on the toes of Physical fusions now...) Giving those elemental debuff abilities to the modifier that cranks up Cunning makes a lot of sense mechanically, and flavor-wise I'm sure we can agree that being set on fire is definitely a curse of sorts.
...Solid is a bit feeble too but more HP and passive defenses rarely do you wrong so I'll cease my Modifier-bullying.
ANYWAY THAT'S MY HOT OPINIONS FEEL FREE TO IGNORE ME