If it is built into the browser, it will not be harder than say cookies. When it comes to US dollars, you have to pay transaction fees and it can take days before the transaction is completed.
The user doesn't need to know what a cryptocurrency is, the dialog will be displayed in dollar, or whatever currency you happen to use in your country.
Today's system is more complicated. You have to create an account, confirm e-mail, remember your password, enter credit card number, address, expiration date etc.
But the user needs a wallet at some point, and the user needs to put money into the wallet so it can be spent. How can that part be hidden from the user?
Today's system may be complicated, but users understand it and are used to it.
You need ONE wallet and then you can use it for all sites. It's much, much easier.
Setting up the wallet doesn't need to be more complicated than setting up a card with Apple Pay. You don't need to understand the underlying technology to use it in millions of stores.
Today's system is like setting up Apple Pay every time you need to buy something in a store you have not visited before. It's a horrible user experience.
I see no reason the US dollar is unsuitable for microtransactions.