This sounds like a great way to quantify policy/law impact on businesses, by comparing their financial performance.

Obviously to an extent it can be compared, but considering laws aren't the only difference (economics of how many people in each country need work at what price, culture of who owns cars vs uses taxis vs cycles, etc etc) it would take an awful lot of analysis to compare them usefully (which no doubt various people both inside and outside Uber are doing, of course - and Ive no knowledge of how successful they are at that).

You couldn't just look at one being twice as profitable and say "that's the effect of country A's legal regulations vs. country B's".

Also I'm not sure the data is available, does the US entity which is publicly traded have to report on all subentities' financials separately? Don't they usually just consolidate the financials?

The individual local business entities have their own mandatory data publication requirements. At least in EU, it's usual for companies to have to share some economical data publicly (usually there's a minimum on revenue but Uber certainly is over).