No ...They are a great keyboard
There were a few issues but these seem to have been sorted with the latest updates.....I had some issues with mine but it was replaced and the new one up-dated was great
I have not seen many used ones.... The few for sale are much the same price as a new one, they are holding their value VERY well.
If i were you i would buy a new if you are worried and then you will have a Yamaha guarantee ....Do be aware, warranty is non transferable to a second owner in a private sale, although a shop may offer you its own warranty.
New prices now are VERY keen for both the 700 and 900, well here in the UK they are.
I actually now own a 700 which for me is fine ....do compare the spec and save the extra money if you don't need the different features.
they really are the same 'basic' board with the same keybed, but there is a masssive price difference now.
I was lucky and managed to sell my 900 without loss ( shortage of units as I understand due to Covid ) and thena couple of weeks later managed to secure a super price on my new 700 as stock seemed to be moving again and the price had fallen by almost 20%
Good luck....you wont regret buying an SX i feel sure! whichever you choose.
Keith