The problem for sign builders is that Igni is so incredibly bland. While the base mode does almost no damage the flamethrower just stunlocks and burns most enemies. Sword combat is incredibly bland too and only needs rudimentary skill, I play on hard and never had problems coming by with both. Another thing is that the sign upgrades are very uninspired and boring(melee too), and that Igni is actually the only real damage source from signs we have because the final Aard upgrade does not scale with intensity and is thus completely useless, while yrdens DPS is too low, but thats also not its main purpose.
Another problem is that the game kinda tries to push us to play a mixed style while some players want to specialize, and in fact the point requirements for skill trees are so high that you have to invest points in skills you dont equip most of the time or dont want just to have access to a more advanced skill thats worth it. So it wants us to mix but forces us to specialize, someone who has a strong igni wont see much of a sense using swords if his igni is already so upgraded that he can roast everything while there are not enough points in swordplay invested to make it attractive.
Same might go for a swordfighter that is always critting and builds up adrenaline at fast speeds, he might not see the use in signs when he can just swoop away everything with a slash or two.
Both styles lack a lot of variety and get boring really fast. Combat is just not deep enough, hoping for more patches and extra content that will spice it up. Right now it looks like a copy from dark souls to me, just with bad hitboxes and way more stunlocking of enemies and only one fighting style.
Seems CDP invested their creativity rather in writing than in ability design. Still enjoying the game, but combat and movements are just its downsides right now.