I don't think there were any major issues with Nova, and there wasn't anything related specifically to Onion.  Nova had an outage a while back, and overall their trading volume is low compared to the bigger players.  For instance, Bittrex has traded over $214MM worth of volume in the last 24-hours compared to Nova's $8MM.  Low overall volume means even lower trading volume for Onion.

There are plans to get Onion onto a premier exchange but overall the community is more excited about the upcoming features such as DeepSend.  Since I'm only holding and plan to hold until the end of the campaign I'm not as concerned about where Onion is exchanged.