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The problem isn't in either of your opinions regarding what is right or wrong or whether people should be catered to or cut off. The fact of the matter is that there is no concrete reason why in-character things should have to be telegraphed in any way. A special attack shouldn't have to be telegraphed any more than a normal one, and even though a jab is relatively light and fast so is a thrust with a bladed weapon but the difference in overall impact on a fight between the two is massive.

The major difference, ideologically, falls in that when someone does a fast thrust with a blade they usually tend to be wielding the blade to start with so that there is a visible cue of the danger that a character can react to. Now, this doesn't necessarily have to be the case if you're talking about hidden weapons and trick attacks and things like that... it's an entire facet of a certain playstyle.

MECHANICALLY, however, there is an issue that everyone needs to concern themselves with regardless of their stance on the issue and that is this:

You cannot, in good conscience, have a sword that bursts into flames when it hits and not mention that fact until after it hits. The reason for this being a simple logistical issue that I will describe as follows:

Person A: Swings with sword.
Person B: Takes a shallow wound from sword, counterattacks.
Person A: "When the sword touches you, it bursts into flames."
Person B: Has to make an entirely new post to reflect this new information, making the entire previous post pointless even though Person A didn't actually DO anything.

All of the pertinent information for receiving an attack should be in the initial attack. If the sword bursts into flame when it hits, but has no outward indicator that this will happen until it happens... then put it in the post. "If it connected, Person B would find much to his surprise that the weapon burst into flame just as it touched him." This way Person B can respond to the entire attack without having to split a reaction up between 2-3 different posts.

If you hide something from the other player OOC, you're effectively telling them "I don't trust you not to metagame, so I'm not going to tell you what will happen if you take this hit until after you take this hit".

This should (and I mean should since my last revision to HoH rules is the only ruleset I know about that actually called people out on this) be something people are aware of as being a problem - that people should not be able to wait until after a hit is confirmed and then decide, after the fact, to apply additional on-hit effects. All of that information should be handed out up-front.

Mechanically speaking.

In-character, you don't know what the ******** is happening until it happens.

I in my earlier ramble post addressed things like a poisoned blade.

It fits into that pentagon of effort, risk, damage (or reward is a better term), resources, and range.

-Effort: Decent. You have to penetrate exposed flesh to make this work, so you need to really bypass the opponent's defenses and range keeping mechanics.

-Risk: High. If your character's centric around poison use and you ******** up applying the poison, your potency just dropped. Furthermore, there's a distinct possibility if things go really wrong of you getting pierced by your own poisonous weapon. That is dangerous.

-Damage: Massive. This can drop someone if not seriously inhibit them.

-Resources: Limited. You only have the one dagger [or a couple] and a limited amount of your poison, which you had to apply before the fight even started.

-Range: Close melee.

Verdict: Balanced and totally fine to be absolutely concealed, barring any external factors we're not going to put into this to keep it simple.

Except, your version of balanced may not be the same for anyone else, nor can you tell them otherwise.