So good for secrecy, bad for deniability. All told, strangling someone and making it look like a suicide, or “helping” someone make a jump is probably still simpler and safer.
If I recall correctly there was a story about a CIA weapon that shot a needle or dart made of a frozen toxin. It would penetrate the skin, melt, and cause a heart attack. There would be no evidence other than an incredibly hard to find hole in the skin with no physical object to be found.
Having said that, such a toxin that could also be frozen and the resulting “dart” being strong enough to survive being fired into someone, is not perfect. Like the poison umbrella, you’d expect traces of toxin to be found around the wound, and even small wounds become evident under ALS.
The real question here isn't the delivery mechanism, but the properties of this magical compound - whether it leaves any traces in the body, and what symptoms it causes.
With the right compound in the needle, thst sounds like an incredible way to assassinate someone without leaving any traces which are unlikely to be found. The combination of a tiny needle mark on the inside of the stomach, a lack of dissolved pills in the gut, and a likely delay between administration and onset of effects, it’s sort of perfect.