Like the 2003 Northeast blackout, for instance. [0] Things like that, where an error in one part of the grid cascades into massive issues elsewhere - those don't ever happen.
Sometimes, being big is a danger. Even when that's not the case, it can be worth accepting the same risk of each individual point on the map losing power if it means that (for instance) Ohio still has power while Indiana's in the dark and vise versa. I'd rather have one state in the Northeast lose power each year than have every state lose power at the same time every 15 years, if that makes any sense.
The main benefit from having one ISO in the US would be a reduction of rules. If you wanted to build solar or gas or wind in California, it would be subject to the same grid rules as Texas (e.g. how to handle a fault on a line - how quickly it recovers from fault, from frequency or voltage deviations, etc.). The fact that a particular power grid is isolated from others means they can't benefit from cheaper power due to overproduction of say wind power in Kansas. That electricity goes to someone else or goes to waste.
Finally if anyone here cared to read, the FERC doesn't regulate the Texas grid.. it's ISO has its own rules (which I'm pretty sure are similar to the rest of the ISOs in the US which are subject to meeting the FERC regulations or going beyond them). The Texas grid is connected to other grids.
"The Texas Interconnection is tied to the Eastern Interconnection with two DC ties, and has a DC tie and a VFT to non-NERC systems in Mexico. There is one AC tie switch in Dayton, Texas that has been used only one time in its history (after Hurricane Ike).
On October 13, 2009, the Tres Amigas SuperStation was announced to connect the Eastern, Western and Texas Interconnections via three 5 GW superconductor links.[2] As of 2017, the project was reduced in scope and only related infrastructure was constructed for nearby wind projects connecting to the Western Interconnection." https://en.wikipedia.org/wiki/Texas_Interconnection
I worked next to a power plant that shared some network components with my employer at the time. They did an emergency shutdown minutes before we were impacted by blaster. Some of our upstream providers operating backbones in NY also were impacted and had capacity issues due to the worms. At least one of those providers had issues for days after the initial incident.
There is indeed risk for cascading failure but it is very rare (how long have the US grids been interconnected vs. how many times has there been a massive, multi-state blackout?) and with EU countries higher attention to infrastructure maintenance the risk is even less.
(And sometimes a system being extremely stable just means it's brittle, not strong)
The design of electrical power grids is such that their control systems are by their very nature suppose to keep frequency and voltage within tolerances no matter the unpredictability of the grid. This is the definition of stability and resiliency that you're talking about. And the design takes into consideration failures of any device on the grid.
That being said, even being a minor power grid, it isn't actually isolated. Its just not subject to FERC regulations. But for the most part, its own regulations are similar to that of what other grid operators are doing, because it makes sense to do for grid stability reasons.
"The Texas Interconnection is tied to the Eastern Interconnection with two DC ties, and has a DC tie and a VFT to non-NERC systems in Mexico. There is one AC tie switch in Dayton, Texas that has been used only one time in its history (after Hurricane Ike).
On October 13, 2009, the Tres Amigas SuperStation was announced to connect the Eastern, Western and Texas Interconnections via three 5 GW superconductor links.[2] As of 2017, the project was reduced in scope and only related infrastructure was constructed for nearby wind projects connecting to the Western Interconnection."
While nuclear plants do have diesel generators for this, it'd still be a good idea to have one or maybe two sections of the country which could bootstrap the rest of the North American grid(s) should there be a continental blackout.
Except when it isn't.
In Europe most of our grids are already connected and there are plans to create a supergrid that stretches all the way from Finland down to North Africa (mainly to get connected to the future Saharan solar plants).
Usually the bigger the grid the more resilient it is and the less dispersion there is in demand.