You typically aren't allowed to tax-free swap one asset for a different asset, even if the assets are of the same type. You can't tax-free swap Facebook stock for Amazon stock, for example, or a gold coin for silver coins of equivalent market value. There are a very few exceptions, such as in real estate where you can do what's called a "like kind" 1031 exchange, or with corporate mergers and acquisitions where you can do a tax-free re-organization, but all of these exceptions require specific contemporaneous paperwork to be filed.

This isn't much of a problem because BTC and ETH are close to all-time highs, but in theory you can see how somebody could exchange BTC for ETH, ETH plummets, and they end up owing more in taxes than they are worth. A similar scenario occurred after the dot-com crash in 2001.