(No longer) used Advanced Custom Field custom location rules for a client project, that might be useful for something else in future (props: Bill Erickson, as usual)

acf-rules.php

<?php
/**
* Register conditional locaton rules for Advanced Custom Fields
*
* @package     ${NAMESPACE}.
* @since       1.0.0
* @author      iamnickdavis
* @link        http://iamnickdavis.com
* @license     GNU General Public License 2.0+
*/

add_filter( 'acf/location/rule_types', __NAMESPACE__ . '\register_rule_type_has_locations' );
/**
 * ACF Rule Type: Has Locations
 *
 * @author Bill Erickson
 * @see http://www.billerickson.net/acf-custom-location-rules
 *
 * @param array $choices , all of the available rule types
 *
 * @return array
 */
function ta_acf_rule_type_has_locations( $choices ) {
	$choices['Post']['has_locations'] = 'Has Locations';

	return $choices;
}

add_filter( 'acf/location/rule_values/has_locations', 'ta_acf_rule_values_has_locations' );
/**
 * ACF Rule Values: Has Locations
 *
 * @author Bill Erickson
 * @see http://www.billerickson.net/acf-custom-location-rules
 *
 * @param array $choices , available rule values for this type
 *
 * @return array
 */
function ta_acf_rule_values_has_locations( $choices ) {
	$choices = [ 'True' ];

	return $choices;
}

add_filter( 'acf/location/rule_match/has_locations', 'ta_acf_rule_match_has_locations', 10, 3 );
/**
 * ACF Rule Match: Has Locations
 *
 * @author Bill Erickson
 * @see http://www.billerickson.net/acf-custom-location-rules
 *
 * @param boolean $match , whether the rule matches (true/false)
 * @param array $rule , the current rule you're matching. Includes 'param', 'operator' and 'value' parameters
 * @param array $options , data about the current edit screen (post_id, page_template...)
 *
 * @return boolean $match
 */
function ta_acf_rule_match_has_locations( $match, $rule, $options ) {
	// Only run for 'has_locations' value
	if ( 'has_locations' != $rule['param'] ) {
		return $match;
	}

	// Only run if post ID is defined and this is a provider
	if ( ! $options['post_id'] || 'provider' != get_post_type( $options['post_id'] ) ) {
		return $match;
	}

	$terms = get_the_terms( $options['post_id'], 'location' );
	$count = count( $terms );

	if ( $count > 1 ) {
		$match = 'True';
	} else {
		return $match;
	}

	return $match;
}

add_filter( 'acf/location/rule_types', 'ta_acf_rule_type_has_specialty' );
/**
 * ACF Rule Type: Has Specialties
 *
 * @author Bill Erickson
 * @see http://www.billerickson.net/acf-custom-location-rules
 *
 * @param array $choices , all of the available rule types
 *
 * @return array
 */
function ta_acf_rule_type_has_specialty( $choices ) {
	$choices['Post']['has_specialty'] = 'Has Specialties';

	return $choices;
}

add_filter( 'acf/location/rule_values/has_specialty', 'ta_acf_rule_values_has_specialty' );
/**
 * ACF Rule Values: Has Specialties
 *
 * @author Bill Erickson
 * @see http://www.billerickson.net/acf-custom-location-rules
 *
 * @param array $choices , available rule values for this type
 *
 * @return array
 */
function ta_acf_rule_values_has_specialty( $choices ) {
	$choices = [ 'True' ];

	return $choices;
}

add_filter( 'acf/location/rule_match/has_specialty', 'ta_acf_rule_match_has_specialty', 10, 3 );
/**
 * ACF Rule Match: Has Specialties
 *
 * @author Bill Erickson
 * @see http://www.billerickson.net/acf-custom-location-rules
 *
 * @param boolean $match , whether the rule matches (true/false)
 * @param array $rule , the current rule you're matching. Includes 'param', 'operator' and 'value' parameters
 * @param array $options , data about the current edit screen (post_id, page_template...)
 *
 * @return boolean $match
 */
function ta_acf_rule_match_has_specialty( $match, $rule, $options ) {
	// Only run for 'has_specialty' value
	if ( 'has_specialty' != $rule['param'] ) {
		return $match;
	}

	// Only run if post ID is defined and this is a provider
	if ( ! $options['post_id'] || 'provider' != get_post_type( $options['post_id'] ) ) {
		return $match;
	}

	$terms = get_the_terms( $options['post_id'], 'specialty' );
	$count = count( $terms );

	if ( $count > 1 ) {
		$match = 'True';
	} else {
		return $match;
	}

	return $match;
}