I downloaded last night around 2am and played an hour on the handheld.
In short- its Rocket League on a handheld. Yes, they made graphical compromises- I'd say if console games had a PC like graphics detail slider- the game is probably set somewhere between the low and mid setting, and we know this is running at lower resolution than other platforms. Everything is muddier, less detail, etc.
But the gameplay is perfect, its running at 60fps, all game modes and content is there. If you wanted Rocket League on a handheld platform, don't hesitate to buy this, as its in all its glory.