Djem So has a lack of mobility, but offer you great and powerful attack. It depends on raw power, and Anakin is the best person for this form, because of his connection to the force allows him to become very strong. Ataru gives you acrobatic move and speed, Shien gives you defense and critical strike, Djem So gives you very-very powerful strike. And martial arts will just improve Djem So effectiveness, because your punch/kick will be very strong, due to Djem So's ability to DOMINATE people.
I've heard people saying that Vaapad is like Djem So, but more deadly. I don't think so. Vaapad feeds on dark side emotion like hatred and vengeance (that's why Mace is very strong). It gives you power, but not on Djem So''s level. Because hatred and vengeance have their limits. Unless we're talking about Hulk. That's something else. And Vaapad also has that superconducting loop thing that makes the form effective against dark-siders.
In other words, people with insane strength (like Hulk/Supes) = cannot be beaten. Sorry for being off topic a little bit.
Ataru's limitation is "tactical" disadvantage (like small room).
Juyo's limitation is anger (yes, it draws strength from anger, but anger has limit).
Djem So's limitation is mobility (because it is focused on dominating enemy).
Luke doesn't use Djem So. He use the fast-medium-strong style of the New Jedi Order. He only replicated Vader's form V (it doesn't look like Djem So, IMO) in that fight.
Any weakness can be covered.
Ataru = small-sized body (like Yoda, for example), so you can move well
Juyo = have unlimited anger like Hulk
Djem So = high Force reserve and good body, so you can kill people easily with good mobility (like Anakin)