I think the more difficult instrument would be percussion, and that's because everything involving the instrument has to be perfect. With winds or strings, a good player can make up for deficiencies in the instrument and still make it sound good. It's much more difficult for a musician to compensate for problems in a percussion instrument. For a percussion instrument, you need to find a material that sounds good for both the instrument itself and the mallets, and for it to be in tune, you have to have very accurate measurements. It's a lot of attention to detail, but it can be done- the last three national champions have had a xylophone or a related instrument.
Of course, you could make a case for any instrument being toughest to make and play. Our own experiences with the event undoubtedly make us biased. With regards to whether or not there will be limits next year, I don't think so. You can never know, but I doubt that the rules will require certain instruments. Let each team figure out what works for them- there will be differences.
On the other hand, I'd love to see them make electrophones mandatory.
![Razz :P](./images/smilies/icon_razz.gif)