The only real downside is that it decreases the amount of control that your power company has over your local grid, which doesn't sound like a downside until there is an electrical emergency and unregulated solar starts dumping into the grid and puts line workers at risk.
At the same time, it is a fairly small downside when 1 person does it, but the effects increase quite a bit when everyone does it.
I'm not talking about islanding, I'm talking about what happens when every home has solar and the grid is producing too much electricity for their need.
Grid excesses are still a possible issue in that scenario, but the power company will not want to turn off the grid as if they go down everyone's grid tied solar does, too.
This won't simply dump power into the grid (unless the used inverter would violate codes right and left), since those inverters need to sync to the grid first before allowing for any output.
So if the grid is down - nothing will happen.
Overproduction is a possible issue for the grid operator. It's not a huge risk, but if there is an issue where the grid is producing too much energy, and at the same time homeowner solar is also producing a lot, then it becomes a potential issue for the grid to have to figure out how much production to cut to minimize the risk of damaging the grid while also not browning out the grid.
It's a small risk, but one to consider when UL listing cheap solar for everyone.
You're talking about islanding protections to protect lineworkers? German regulations require them. AFAICT The NPR article points out that the UL Solutions certification will cover exactly this.
Not islanding, overproduction, where the grid owner is trying to accurately manage decreasing production to match need.
Solar is more difficult for this as when a grid is largely solar the local producers and grid producers will tend to have simultaneous excess and need in synchronous day/night cycles, so the grid operator would need as much accurate and real time data as can be had so that they would know how much and when to charge batteries or to otherwise dispose of unneeded waste electricity for their local area.
At the same time, it is a fairly small downside when 1 person does it, but the effects increase quite a bit when everyone does it.