I have four eero gen 2 devices and a beacon in a very large home.   One of the eeros goes offline just about every day for a few minutes then comes back on. When it happens my entire WiFi is crippled for that time period.  Directly connected computers (not on the mesh network) continue to work.

Support has been trying to resolve the problem but I read more and more similar complaints and have come to the conclusion that they can't help me with this problem.  It seems to be something inherent in the mesh system.  

Sounds familiar.  I swapped the disconnecting main eero with another one.  Made no difference.  

Support had me jumping through hoops where they wanted me to disconnect much-needed hardware from a switch I had in place to narrow down the problem.  At that point, I just gave up.   If I have to go through much trouble to find out why these eeros do constant short-term disconnects then perhaps I should look to upgrade with another brand.

I have 9 eeros in a large house as well and notice IDENTICAL issues.  I have also called eero about this and they have told me similar troubleshooting steps.  I am not sure about your setup, but I have 6 of the 9 eeros wired to ethernet that routes into a 10GBE switch in my basement.  The other 3 eeros are in spots in my house that do not have access to ethernet (I am working on this though). 

Anyway, I have noticed the exact same issue -- once in a while -- seemingly at random one of the eeros will drop offline.  This one eero causes the whole system to have a fit and one by one the eeros will all disconnect and reconnect.  Sometimes, just a few will.  I can not figure it out or what is causing it.  I had this setup running for many years with no issues at all, but about a year ago is when I started to notice it.  I have also noticed that certain updates are worse than others about the frequencies of eeros dropping offline.  

I am finding this very annoying -- especially with work at home requiring stable internet, intranet and wifi connections.

I am curious what others experiences are with alternative mesh style devices.  I really did love eero until about a year ago when this started.  More recently I am also doing my homework as to what I can use as an alternative.

I have two of the Eeros connected by underground 200' CAT 6 ethernet that goes from one end of the house to the other.

This does create a much stronger WiFi infrastructure. 

Part of me is wondering if so many Eeros (I have 5 Pros plus a beacon) in the mix is killing the connection?

The other part of me sees similar complaints from other owners and the inability for Eero support to come up with an answer or fix.  All I get from them is wanting me to start unplugging necessary equipment from switches connected to their mesh devices.  

The problem has become so frequent that I am looking at other mesh vendors when I plan to upgrade to WiFi 6 or (eventually) WiFi 6E.

Yep. Same exact Issue here.

I have 4 x EERO devices around a 4 bedroom semi-detached house. Randomly, one specific device keeps going offline and connectivity is lost.

I've tried resetting the device, removing it and re-adding to the network. Nothing seems to be working.

my issue turned out to be a smart clock that was storming the network,  It was not on my network.  I actually just forgot to connect it once I bought it.  As soon as I added the clock the issue went away.  the eero does not have the ability to block by MAC address.  the tech could see it, but could not do anything about it.  He said this happens with smart lights and devices.  we bought the clock and just plugged it in with out setting it up.   the clock just kept trying to join its closest eero device and took it offline and then disable the entire network.   Hopefully they fix this.

Any update on what was causing this?

I have the same issue.  I have 9 AP's and 3 Pro's.  One of the Pro's was losing a connection and showing Offline so I removed it from the network.  It is still removed but now another one has started to do this.
 

I've been using eero with my Balance 305 for years, but lately eero has had issues where the AP's randomly go offline even though all 4 of my connections are up. It's going to take some serious time to troubleshoot and I'm not sure it is worth the effort or if another system like Orbi is the best option to go with.

My house is 2 story 5800sqft but it has 10" thick walls with thick plaster and exterior walls are about 16" thick. To compensate for this I currently have 10 AP's so that exterior wifi cameras work. The house is old and in hindsight I should have ran ethernet before blowing in insulation last year.