Dynamite Blast is the biggest "noob trap" card in the game, and by that I mean this:
Imagine you just bought the game, know very little about it and decide to get started with the Core Campaign. You pick Roland as your Investigator, as naturally Guardians are usually the Investigator that attract greenhorns the most for solo play, and decide to build a deck yourself. You pick cards to build your deck, see Dynamite Blast among the main offensive events, pick it up and try using it... And then realise how botherline useless it is.
Let's talk about the positive: 3 AoE damage that doesn't not require a test to deal is extremely good. If you nail a room of 3 enemies or so, it already pays for itself despite the high resource cost. The cards also gets a discount and makes you immune to attacks of opportunity when played when upgraded
As for the negatives: you provoke attacks of opportunity when played and it can cause friendly fire, these two combine makes it impractical to blow your location up with it, because most of the time it will leave you and your companions in pretty awful conditions, making any kill you score with it a phyrric victory at best
And now let's talk about what actually makes the card nearly useless in my opinion: it is an issue with the card itself, is an issue in how the game is designed, specifically about one aspect of the game: enemy spawning
Enemies in Arkham spawn usually in 2 ways: on top of you, or in an empty location (connecting or otherwise). Sometimes they spawn the furthest location from you, very rarely on a specific location (that is usally reserved to Elites), but the mayority of spawn instruction are usually right in your face or annoyingly close and separated.
That means, it is rare for enemies to clump up in any location beside your own, if all Investigators are in the room: which is NEVER when you want to chuck dynamite. You never get your dream scenario of blowing up 3 Acolytes and their leader with one explosive because they all spawn separately. You never get a chance of annhilating every Ghoul Minion on the board without first exploding alongside them.
This is a perfect example of a card that would be better if it belonged in a different class: Rogues can make much better use of this because at least they can run away from enemies using Elusive or Cat Burglar and then chuck a stick of dynamite in the room just left, but even you run into the issue of having to evacuate your buddies as well to avoid frindly fire. Still "Skids" and Jenny can take it, and Skids can even upgrade it to level 2, if you are considering running it to make up for his average fight.
I do not think Dynamite Blast is a good event: it is not completely useless, but most of the is impractical to properly use in most situations, and the few time where you can get that perfect scenario where you get to blow up 6 enemies without hurting any Investigator means you just failed your job as Guardian by allowing these many enemies to be alive at once.