Glycerin can throw off your natural pH and lead to irritation or yeast infections.

Some condoms still use spermicide, which studies link to skin irritation. Source: CDC

Some add synthetic fragrance to mask the funky smell of casein (a milk derivative).

Some contain benzocaine to act as a numbing agent, which reduces sensitivity and delays ejaculation.

Found in chili peppers and used to create a warming effect, capsaicin can easily cause burning, redness and discomfort in the vagina.

None that are required to make sex safer. It's just what's cheapest to manufacture at scale.