Proof of concept for avoiding admin-ajax for ajax callback requests. Also see Thomas Griffin's excellent post: https://thomasgriffin.io/a-creative-approach-to-efficient-and-scalable-wordpress-api-endpoints/ AND Josh Pollock's excellent post: http://torquemag.io/improved-wordpress-front-end-ajax-processing/

ajax-endpoint.js

jQuery(document).ready(function($){
	$('body').on( 'click', '.some-button', function(){

		$.ajax( ajax_endpoint_data.api_url, {
			type     : 'POST',
			dataType : 'json',
			data     : {
				action: 'ajax_action',
				some_data: 'some_value'
			}
		}).always( function() {

			// Hide your spinner

		}).done( function( response ) {

			if ( ! response.success ) {
				// Display error message, ajax_endpoint_data.error_text
			}

			// Happy Path! Do your success message

		}).fail( function() {
			// Display error message, ajax_endpoint_data.error_text
		});

	});
});

front-end-ajax-callback-api.php

<?php

class Ajax_Endpoint {

	/**
	 * The nonce key to verify if we should do our ajax api endpoint action
	 *
	 * @since    1.0.0
	 *
	 * @var      string
	 */
	public $api_nonce_key = 'ajax-endpoint-nonce';

	public function __construct() {
		// late enough that cpts/custom taxonomies are registered
		add_action( 'init', array( $this, 'ajax_endpoint_api' ), 99 );
		add_action( 'wp_enqueue_scripts', array( $this, 'enqueue_js' ) );
	}

	/**
	 * Detects if we're trying to make our front-end ajax api call
	 * This can occur even higher in the load the stack if you don't
	 * need custom post types or custom taxonomies
	 */
	public function ajax_endpoint_api() {
		// are we on the api endpoint?
		if (
			// Do we have all our request vars???
			! isset(
				$_SERVER['REQUEST_URI'],
				$_REQUEST['ajax_action'],
				$_REQUEST['some_data'],
				$_REQUEST[ $this->api_nonce_key ]
			)
			// Are we looking at our front-end ajax api endpoint?
			|| 0 !== strpos( $_SERVER['REQUEST_URI'], '/ajax-endpoint-api' )
			// Are our request vars empty???
			|| ! $_REQUEST['ajax_action'] || ! $_REQUEST['some_data']
			// Did we pass the nonce test?
			|| ! wp_verify_nonce( $_REQUEST[ $this->api_nonce_key ], $this->api_nonce_key )
		) {
			// Looks like we failed, so don't do anything (probably will land on a 404)
			return;
		}

		// Now you can do your business logic
		if ( $some_condition ==  $_REQUEST['some_data'] ) {
			wp_send_json_success( 'You did it!' );	
		}
		
		wp_send_json_error( 'Condition failed. :(' );
	}

	/**
	 * Enqueues/Localizes
	 * Pay attention to the api_url, that's important
	 */
	public function enqueue_js() {

		$min = defined( 'SCRIPT_DEBUG' ) && SCRIPT_DEBUG ? '' : '.min';

		wp_enqueue_script( 'ajax-endpoint-js', $plugin_url . "assets/js/ajax-endpoint$min.js", array( 'jquery' ), 'version' );

		wp_localize_script( 'ajax-endpoint-js', 'ajax_endpoint_data', array(
			'debug'        => ! $min,
			'api_url'      => wp_nonce_url( site_url( 'ajax-endpoint-api' ), $this->api_nonce_key, $this->api_nonce_key ),
			'error_text'   => 'Error Text',
			'success_text' => 'Success!',
		) );
	}

}