OK, so how powerful are Rahkshi? Equal to the average Toa?And how some some Rahkshi control elements (ex. Su-Rahkshi) and some control just random powers?
It was hard for the Toa Nuva, the ultimate Toa, to defeat them, but later in the story they are easy to subdue. Maybe the six Rahkshi used on the Toa Nuva were the most dangerous?
To add to previous posts (and let me give one giant nod to Katuko's post), the 2003 Rahkshi combined every factor possible (or nearly) to give them the maximum total effectiveness in that situation. A mission (motivation rule), level (strength rule) and good power matchup against the Toa they were facing at that time (until Takanuva came along, and then they were easy to subdue). Plus direct oversight by Makuta, and staggering the launch, so three came out first, then delay, then three more, as well as sneak attacks and the like.
Rahkshi later on were often either encountered randomly (2004 flashbacks) or were in the final battle, so geared up for "attacks in general" rather than to a specific small group of characters.
So statistics demands that in most of those situations, Rahkshi should be easier to defeat. The random encounters with wild Rahkshi will by definition lack motivation, and will tend to be lower level. Whether they're a good power matchup would be random. They're also usually less in number.
In the battle, the Rahkshi may have been more limited than the 2003 Rahkshi in some ways just by virtue of being in a big group, including non-Rahkshi allies. They'd have to tone down their use of powers to make sure they didn't accidentally kill/defeat their own allies or other Rahkshi getting in the way. So a big group would go up in danger level dramatically for a while, but over a certain size it would seem to level off. Also, to populate the battle fully Makuta likely had to send whatever Rahkshi happened to be ready, including mostly lower level ones, and Makuta wasn't directly overseeing them as he was distracted by Mata Nui.
Where is the line between elemental power and just power? Rahkshi can have elements or just powers, and so can Kanohi, so where is the difference?
Keep in mind that in 2003 when the Kraata powers list was given, those Kal elements were not elements yet; Greg only decided that later. And it's possible to have a "just a power" version of an element, rather than a full Toa-style elemental control. So the Rahkshi versions are almost certainly "elemental sub-powers".
But in general, the difference between an elemental power as in what Toa have, and just a power is that elemental power does not specify how the element is used, so the user is far more free to use it however they imagine, and it's a bit more flexible in terms of power too; they can move existing elements, reshape them, or make new, or absorb existing (temporarily). None of that, especially the imagination requirement, is a good match to your average Kraata IMO.
"Just a power" by contrast has built-in rules that are usually very specific as to how the power works. For example, the Kanohi Garai, the Mask of Gravity, appears to be a subpower of Gravity that has special limits on it that a Toa of Gravity would not have. The psionics sub-power of telekinesis with the Kanohi Matatu is another great example, or one of the eight basic Kanoka powers of Freezing as a subpower of Ice.
In the case of a Rahkshi of Plasma, Electricity, or Sonics, the easiest ones to figure out probably, I would presume it's just "shoot elemental energy which turns into the element immediately". So they would only be able to shoot bolts/beams of the element (or so the theory goes). That of course gets tricky with Gravity and Magnetism, but I think they would just shoot the force out, attracting matching objects toward them (gravity weakly attracting everything toward the Rahkshi, and magnetism strongly attracting just metal and/or magnetic rock).