You are right, CG, but IMHO you have picked the wrong comparisons.
The new aviation diesels are among the least reliable engines there are. Maybe Rotax are even worse; I don't know. The old Lycos suffer from a poor thermal design (requiring specific operating procedures which are not taught) and nowadays poor QA, and all this translates into big suprise bills at the next Annual, but catastrophic failures are extremely rare.
The reason why there are so many niggles on the training fleet is that it is generally maintained to the minimum legal standard. A school will have a favourite maintenance firm, often many miles away (and they sometimes get a PPL holder to ferry the plane there, at a discounted rate) which does what one might call the required work and no more, at the right price. I would do the same if I ran a school. You don't want the plane to actually fall apart, but why spend too much? Fixed wing stuff is pretty safe; short of a wing falling off there is almost nothing that should be fatal.
On my first trial lesson, many years ago, I noticed the radio being held by just 1 screw; the other 3 had fallen out. INOP stickers are common today, but for VFR that's OK. Often, you don't even need a radio.
The build quality on turboprops and jets (I see their innards almost daily) is wonderful in comparison to piston GA but also their owners tend to have loads of money and get stuff fixed. A 50hr check on a big old turbine twin might cost £20k. It's a different world.
Basically, most of the schools are running Mk1 Cortinas, the original ones, and they change bits as they pack up.
Cars are reliable today, even the cheap ones, but partly for different reasons. It's true that electrics has improved massively (in the 70s it used to account for most breakdowns) and the plane business has not generally learnt that yet but also the engines run at some 10-20% of rated power most of the time. Even 70 mph is perhaps 40% power, in a family saloon. IMHO, the reliability improvements in cars in recent decades have come mostly from attention to corrosion damage on electrical and mechanical parts.