There are people who have far more knowledge than I do on the subject of Japanese Tantojutsu. The Tanto was a rather large knife and in some cases, so large that some people NOW would consider them a short sword. With the larger Tanto, if held in reverse grip with the blade lying on the forearm, the point could be about two inches past the elbow or so. And that was considered "Tanto."
There are many different variations on these words and it almost takes a historian to know precisely what any given knife is. In the WW2 Vs. Modern Arts thread in Practical Tactical, I danced around with some stuff that is in this realm as it is an interest of mine. I like to steal ideas from other systems, etc.
Tanto had a full sized guard, Hamidashi had a small guard. The Aikuchi was always guardless as far as I know. Kwaiken could be a man or woman's concealed knife and were often chisel ground. Then Yoroi Toshi which were armor-piercing short knives.
Tantojutsu, from what I have seen, is very viable. I have nothing bad to say about it at all. It is just "different."