This is not only a valid, but also an extremely important question for a beginner to ask who is interested in moving to a new instrument. I did not ask for an easy theremin to play. None of them are easy to play, but I can guarantee you that some are easier to play than others for a variety of reasons.
I can't think of one instrument that does not have models with playing differences, some extreme that can eventually boil down to a discussion of "playability". The theremin is no different. And perhaps it's even more important to ask this question of the theremin because you just can't walk down to your local music store and try out all the theremins you might be interested in (if they even have one model).
I have found from the limited number of theremins I have played that there is actually a very wide and marked difference in playability ("ease" if you will) between them. This seems to involve a combination of factors - linearity, response (both pitch and volume) and to a lesser degree tone and output level and of course physical design (some theremins may not be as easy to play taking height and arm length into consideration).
For example, I have an Etherwave Pro and a Burns B3 Pro and to me they are completely different animals. The Burns is much less responsive. Can't get as crisp and fine a vibrato or tone. But the end result is that the Burns is a heck of a lot easier to play in tune as the Etherwave will fluctuate in pitch more so improving vibrato technique becomes more important to play in tune on the Moog. For some types of music, that may well be an important factor to consider. So to me the Etherwave is capable of more expression (but it also translates in my mind as being harder to play). I like them both, and I actually might play different music on them right now (I expect even professionals might chose a different instrument based on the style and difficulty of the music they are playing).
So I would expect the people who have become proficient and who have played a wide range of theremins will definitely be able to pronounce some instruments are "easier" to play than others. And for people like Luna, who are signing up to buy an expensive instrument based on hearing a few internet recordings of someone else, this may well be a very important question (especially because not all Subscope theremins may well play the same way so quantifying things a bit may be very helpful to her so she can better tell the instrument maker what she is looking for).
Though Amey says she "doesn't know why" the Subscope is best for her, if she thinks about it a bit, I bet she can quantify more why that Subscope is so great for her (and of course just because she prefers a Subscope may not mean everyone will). It would be interesting to hear Amey discuss what about each of her three instruments she likes and does not. Why is the Subscope "easier" for her to play? How is the response vs. the others? Try a vibrato on all three and discuss the difference. Is it easier to play in tune? Does it have better response over all octaves? Is it more tone she loves? Tone can affect you greatly. This is in no way saying the Subscope is easy to play. I bet Luna would be very interested in that discussion.
Then there are folks like Peter who have probably played 10 or more different instruments at length. What are the pros and cons of these? Any student at some point that gets some proficiency will get a hankering to purchase a "better" instrument. With the theremin, unfortunately you have to take a lot on faith as there are few choices and most you just can't try out so you want to gather as much information like this to make informed decisions as you can.