Your SCSS files do not seem to be compatible with bootstrap 4 even though I swear your marketing info/documentation says it is. Trying to compile your SCSS files with Bootstrap 4 triggers a stream of errors about missing variables.
EDIT:
After further digging, I have this narrowed down to a sass/webpack issue. Because you include both the css and the scss in the source files, webpack does not know which file to use since you don't qualify underscores or extensions in your SASS files. This is the output of webpack trying to compile just the button scss:
//my scss file
@import "../../node_modules/@syncfusion/ej2-buttons/styles/bootstrap";
//error
ModuleBuildError: Module build failed (from ./node_modules/sass-loader/lib/loader.js):
@import 'ej2-base/styles/bootstrap';
@import "definition";
@import "bootstrap-definition";
@import "all";
^
It's not clear which file to import for '@import "ej2-base/styles/bootstrap"'.
Candidates:
ej2-base/styles/bootstrap.scss
ej2-base/styles/bootstrap.css
Please delete or rename all but one of these files.