Picoscopes have a lot of good features but for what you want to do they may not be ideal.
Firstly, I don't think that it is isolated because it needs to communicate via USB so its ground point is that of the USB ground. Secondly they don't have as large a voltage range as standard scopes - they are limited to 20V I think though they are protected against higher voltages.
That particular model is cheap and handy to use but only has one channel which you may find rather limiting.
Having said that, most scopes are grounded rather than floating unless you power them from an isolating transformer which is probably not a good thing to do - better to power the device under test that way. And you're only wanting to look at 5V and 12V rails so the 20V limit won't be that restrictive.