Think they're fine, safety-wise. My brother's children (3 and 5) were given one by a neighbour whose child had outgrown it (aged 10), and it went extremely slowly (much slower than walking pace) so was no danger as such! My boys were very very jealous, and there was much excitement about having a go.
However, I think they could be an enormous PITA for you. I wouldn't enjoy letting mine play on a pavement near any road (what if they lost control and drove in front of a car?), so you'd have to take it to the park, unless you have a ginormous garden.
And what if it runs out of batteries? Last Christmas I felt really sorry for one poor dad stranded in the park with a toddler plus enormously heavy ride-on car which wouldn't go.
Lastly, I would consider whether you really want a massive great ride-on car in your garage? My brother (who was initially delighted with the cool electric car) now loathes it because it's so inconvenient and needs charging up so frequently.
However, I realise I sound like a total killjoy. You probably have a much more powerful car in mind for your 8-year-old, and I'm sure he'd be so over the moon with it (who wouldn't!).