The question is...did Burisma act in an honest fashion and report his real income?
Under normal curcumstances it doesn’t matter. It would be on Burisma accountants.
Another point Ukraine and US signed a treaty on ‘prevention of double taxation’ mulled by infamous ambassador Yovanovich. I didn’t study it thoroughly but it might consist rules on options for taxpayers to pick jurisdiction. In that case Hunter could ask Burisma not to deduct his taxes from his income claiming he would post it back home under US jurisdiction. Such a move would let him keep the money but to pay it in US or not is up to his conscience in this case.
Re Hunter’s income:
Burisma paid Hunter’s firm. How much Hunter’s firm pays him, and how much it pays his associates, is in the firm’s records.
If the firm is registered in the US, it has to file tax returns. But, it could be registered in a tax haven.
However, as an American citizen, Hunter Biden (and his associates) have to file tax returns detailing the funds paid to them. Which, should be most of what Burisma paid, since its likely that the partnership (Rosemont Seneca, IIRC) has minimal overhead.