If $user['display_name'] works just fine, then perhaps this will to do the trick. At least it should:
<h3>Asistentes</h3>
<div>
<?php $users = get_field('asistentes'); ?>
<ul>
<?php
foreach ($users as $user):
$roles = new WP_User( $user['ID'] ) ?? false ;
$roles = is_object( $roles ) ? $roles->roles : [] ;
?>
<li>
<p><?php echo $user['display_name'];
echo 'User roles: ' . implode(', ', $roles) . "\n";?></p>
</li>
<?php endforeach; ?>
</div>
