[Coach Chomycia]

There is no such thing as the best strike, just the one that works at the time.

I would like to clear up the difference between Casting and Whipping because there is solid science behind why casting is preferred by RMAX.
The first is the difference in the amount of joints incorporated into the movement. Casting utilizes each joint to add torque to the strike while whipping strikes tend to utilize only a few. The distinction is made on the video.
The efficiency of casting striking is further increased beyond the joints when you take into a account that casting creates more fulcrums and leverage force just because it contains better quality of movement.
This is why casting in preferred by RMAX because in a fight you most likely not going to be on balance so you must learn how to get the most out of your movement in order to strike well while off balance.
There is one more advantage to a casting strike and that is since your movement is more sophisticated and the energy more dispersed through your joints, this type of striking is more sustainable and less likely to cause the practitioner problems later in life. For those of us who want to train for a long time that is an important factor considering that the longer you train the more likely you are to accumulate injuries.

Regarding,

Yes, casting are good strikes, but I've felt whipstrikes that were twice as hard.

Is it possible that you have felt whipping strikes that are harder than casting strikes?
Yes, if you were being hit by someone who was better at whipping strikes than casting ones. Everyone intuitively knows how to punch and whip their arms, casting requires practice just to be able to perform, Like Jack Dempsey said in his book "Great punchers aren't born they are made"(Paraphrased) If you talk to some of the more experienced strikers even the ones who do whipping strikes about their technique you'll find that across the board they have started using more joints and movement with in their strikes to make them more efficient, useful, and sustainable.
All you have to do is look at the difference between amateur boxers and seasoned pros to see the difference.

As far as these comments are concerned,

Also, he bashes whipstrikes and emphasizes casting.

I find this comment has no relevance other than to try discredit someone's point of view on a particular strategy. Does Coach Sonnon prefer Casting to Whipping? You bet he does! But he doesn't just go and bash a particular MA strategy for no reason. He was trying to describe his point of view on the subject. I also know from direct experience that if it came up in a fight and whipping was the appropriate strategy then that's what he would use despite what he prefers.

However, some of the strikes and movements he does are too "Boxer-esque" and prone to creating a situation when you square off of someone (Systema tries to move on multiple planes).

I can see where you could get confused on this one, most of the tape is filmed inside a boxing ring. Boxers by the way move on multiple planes, in fact it is impossible for human beings to not move on multiple planes. Boxing is a limited combat sport, which is why it looks limited(less overall movement) compared to real fights or MMA fights.

Also, he seems to tense parts of his body (shoulder, for example in casting) while moving and creating strikes, which in Systema we try to avoid.

Tension in striking is necessary to minimize trauma in the joints, the question is how much and when.

Finally the original question,

Scott Sonnon explains in Fisticuffs that "casting" strikes can be used for striking and grappling simultaneously without distinction. Can anyone explain this?

An explaination is going to fall short of seeing and feeling; but I'll try anyway.
When you grapple you use your arms, and hands in certain ways. When you hit people you use your arms and hands in certain ways as well. Many people see these as 2 very different things but what is it that they have common? Arms and Hands! So you are using the same hardware for 2 different uses, the next logical step is to use the same movement for multiple uses. What RMAX does for its clients is help to guide them along to more efficient masterful MA performance faster, by programming it directly into the education.

Talk to you soon.

Coach Dan Chomycia
RMAX Instructor