Well,
CQG pretty surely tells you in clear terms their system may not be available all the time AND they have / may have an operations team available 24/7.
Plus they are probably a LOT more redundant than you are - how many computers do you plan to run your trading system on? How good is your data center connected? What do you do if your precious computer running the trading system fails... not that power supplies never blows. I had exactly one incident like that some weeks ago where a power flucturation that should never have happened crashed half the servers in my cluster.
But this is not exactly important because...
...they only do order handling, no trading decisions. First thing when I open accounts for electronic trading is to make sure I know how to handle my orders WITHOUT electronic trading:
* Get the phone number of the emergency order desk and operations team so that they can shut down my account access and close all positions on demand.
* Make a test to make sure they know about me, how to identify, numbers work etc.
* Do not run without external monitoring. This is either me or someone from my company sitting at the desk making sure everything is fine, or a number of external systems (running on at least 2 different locations) checking things. Note that "computer reachable" MAY not be good enough, but it is a minimum baseline. Internet connections and computers fail.
You on your side are different. You make the trading decisions. CQG etc. can easily say "hey, this is JUST a cheap way for you to route orders, but we have other avenues and at the end your trading decisions are your responsibility, prepare". You can also be pretty sure that they will hav SOP in place and their systems are monitored properly.
You can not say that to investors into your automated trading. Because you are where the decisions are made and the backup mechanisms should be in place. And unless you tell me upfront sincerely about the risks.... i WILL take you out for gross neglect. Simple like that. Or at least try, and believe me, you do NOT want to have to defend against that.
Btw., do not run Linux or Windows unattended

Both systems are not suitable for such use per definition (Linux: not certified at all, Windows: MS says so in the EULA), without being monitored
Btw., make sure your attornes knows about IT issue. Because the definition of standard operating procedures is not exactly something most attorneys will know (they know the LEGAL definition, but they won't know how the standard is because they are not in the IT area). But a skillfull lawyer will show hundreds of examples of other companies where critical IT system operations are always monitored by humans permanently to establish standard operating procedure. Including your data center which will have engineers at hand... 24/7 in case something happens to their infrastructure.
Unless CME Globex + ZenFire/Rithmic has 24/7 administration monitoring their (automated) backend systems taking my order, it seems like you're saying I can sue them for gross neglect as well.
* I am sure CME Globex has people on site whenever they run (not necessarily 24/7 - they are not available 24/7, and even if you take in the sunday "cleaning" they do, that leaves saturday without operations). Remember that a lot of things go in in Globex systems likely during off-time anyway (cleanup, maintenance, database loads, analysis etc.) - simply because the systems are less stressed then. You dont do things like that without overlooking. Gven the tremendous amount of people and income Globex has running around, I would be tremendously surprised if their admin staff would not be a on 24/7 schedule. They DEFINITELY would be suable for gross neglect for not having this as standard operating procedure. Including having backup data centers etc.
* I am pretty sure Zen-Fire has no people on site. This is like 90% sure.... because to my knowledge...
* Zen-Fire operations is managed by Rithmic. That said, I am pretty sure Rithmic has people on operations whenever they need to be there.
That being said, though, you should read the Zen-Fire / RIthmic risk disclosure carefully. They pretty much WILL state the system may fail and you have to prepare for that case. Easy for them to say so - they just route orders. Different for you, where trading decisions originate. Basically they can tell you sincerely that you should have an alternative. What is your alternative to your automated trading system failing? Telling customers you were sleeping and they lost money because you were unprepared?
Would be nice if Rithmic would have some additional functionality, and one of them would be a position dead man switch. Ordes / their positions tradked with a "trade ID" and if a trade expires all roders and the position is auto closed. The idea is that a separate API call (like once per minute or once per 5 minutes) sends them the trade ID's that my trading system considers active. Typical dead man switch (my system goes down, they close out things I may have forgotten).