It also makes it hard to centralize type coercions and default values.

Most programming languages vend environment variables as strings. This leads to errors like so:

use :ENABLE_SOUNDTRACK, class: :boolean

access to typed ENV-variables (integers, booleans etc. instead of just strings)

Note: as for setting boolean variables: not only are true/false and 0/1 acceptable values, but also T/F and on/off. Thanks, coercible!