This doesn’t sound like normal understeer by the car’s setup… but more like hydroplaning.
You can get into similar understeer situations, caused by the road’s and/or tires bad condition, even using an oversteery “setup”.
It is already mentioned properly why in normal driving a setup that doesn’t give out the rear sooner than the front, is safer.
Meh, I don't think it was hydroplaning as much as just a loss of grip. I don't think I was going fast enough for that (and the road was at best, a very damp surface - it hadn't rained since the night before.) In fact, it was only still wet there because of the trees. It was dry on the rest of the road
With regards to the old tires discussion... I bought an extra set of wheels to put my race tires on, and they had some old all seasons on when I got them. Plenty of tread left but they were 5 years old....it was baaaaad. Seriously dangerous. About as much traction in the wet as there is in the snow with a new set of all seasons.
With the oversteer vs understeer discussion...when I'm driving enthusiastically, I do enjoy a car that'll oversteer when I want it to. But I do agree that understeer is safer in 99.9% of situations for 99.9% of drivers...
I have a the same car almost the same year. Subaru cars normally do have understeer, even the WRX models.
The Outback/Grand Wagon is a great car but its not a road car, more a hybrid. It rides a lot higher than the other legacy models and has a soft setup for offroad. I have one because we have a lot of dust/gravel tracks around NZ. I also take it down to the gravel river beds.
I always thought it gave a lot of warning near the edge so get your shocks checked. Also remember it reacts slower than a road car because of the soft setup. If you want overtseer just induce it.
The normal tyres it has are all season tyres which sit between standard and snow tyres, they have less grip in the wet but feel quiet good on the dry roads, dust and gravel etc but a little worse than normal on wet surfaces.
Warning not to use tires with different diameter is actually very true, it is very easy to destroy awd system if you have more thread left on some tires.
Reason for that is that when front and rear tires spins at different speed there has to be slip somewhere and it is weakest link that will slip, causing it to fail and it goes fast, 2000km can be enough.
But I think you know this, just thought to post just in case