They are usually discussed as one asset class. They behave nothing alike.
Grouping all trading cards together obscures the most important distinction in the category. Vintage and modern issues are exposed to different risks, respond to different information, and require different valuation approaches.
Vintage: supply is settled, demand is generational
For a card printed decades ago, supply is effectively fixed and condition is the dominant variable. The risk is demographic. The collectors who assign it cultural weight age, and it is an open question how the next generation values an object tied to a career they did not watch.
Modern: supply is known, outcomes are not
For a contemporary issue, print runs are often disclosed and condition is frequently pristine. The dominant variable is the subject's career, which can change abruptly and in either direction. A modern card is closer to a long-dated bet on a person than a claim on settled scarcity.
A portfolio holding only vintage is exposed to a generational question. One holding only modern is exposed to a handful of careers.
This is a large part of why Foil is designed around documented era limits rather than leaving the mix to discretion. Diversifying across both profiles does not remove either risk, but it prevents the portfolio from being a concentrated expression of one of them.