Double sided materials have problems with inverted tangent spaces and other “math” bits.
Also, if there’s any transparency involved, double sided materials generally generate more artifacts, because triangle sorting has much less to work with (this depends specifically on data prep, so may not apply here.)
In general, triangle count is not a big deal in modern games and engines. That doesn’t mean you can expect to drop in your 5,000,000 character into a crowd of 200 people, but it does mean that the technical reasons to not use double-sided materials probably weigh heavier than some triangle count savings for an asset like this.