Strange,
When installing Yii2 with Composer, "vendor/bower-asset" folder is created with "bootstrap", "jquery", "jquery.inputmask", "punycode" and "yii2-pjax" subfolders.
Composer respects "vendor/bower-asset" also when installing other "bower" libraries like "jquery-ui" or "typeahead.js". Actually - this is probably decided by the fxp/composer-asset-plugin v. 1.0.3.
What is confusing, is that Yii’s @bower alias is natively fixed to “vendor/bower” rather than “vendor/bower-asset”, so appropriate files cannot be found by the AssetManager and exception is thrown.
That can be overwritten of course on the bootstrap component level, but I believe default alias value should reflect default bower asset folder name.