Multiline expressions cannot be entered at the REPL when using electric-pair-mode.
Geiser uses the parens to determine if an expression is completed (parens are matched), but that conflicts with the very useful (especially for Lisp languages) feature of Emacs called electric-pair-mode: The expressions are always deemed complete by Geiser and evaluated as soon as pressing RET. This makes it impossible to enter multi-line expressions at the REPL.
Suggested behavior: Expressions should be evaluated only when the cursor appears at the end of the expression.