I also use Ninja 6.5 for autotrading; started with equities 2.5 yrs ago, moved to futures 1.5 yrs ago. I trade around 12,000 sides a month currently, so cumulatively more than 100,000 overall.
I agree there are quirks. I believe there remains a race condition in the code... There are times (once a week?) where order quantity goes wrong, or an order just isn't submitted. But you gradually build up checks that allows you to be aware of these issues.
With futures, I've traded over Pats (horrible), IB, ZenFire... And ultimately settled with Rithmic. I am currently planning to explore TT as well, both as a backup and as a mechanism for more overseas instruments.
I always intended to replace Ninjas order execution code with calls into either QuickFix or Rithmic /TT API... Thinking that would be faster or more reliable; I could just leave Ninja as a backtest tool, quote parser, etc. But 2 yrs into this, I just haven't found it necessary to make the change. Im satisifed with my results, and if it ain't broke...
And btw, I've probably placed up to 10-20 trades a minute during peak. It helps I'm only looking at 8-12 futures instruments, rather than hundreds of equity symbols. I also kep my event-based code as simple as possible.