kanedo

This widget displays all comments on all blogs on your multi site wordpress installation. You can define how many comments should be displayed.

Get All Comments Widget

Creates a widget which lists all comments across all sites of a multi site wordpress installation

CAPitalZ

This plugin creates four (4) seperate RSS 2.0 feeds from posts, comments, pages, and one combined [posts & pages] across all blogs on your site.
Works both on WPMS upto 4.9.20
This will skip the first comment and page of a blog; also will not include private, spam, mature and deleted blogs.
Both Subdirectory and VHOST installations are now supported. Thanks to DaryL.
Features:
<ul>
<li>Generates sitewide combined [posts & pages] feed</li>
<li>Generates sitewide posts feed</li>
<li>Generates sitewide comments feed</li>
<li>Generates sitewide pages feed</li>
<li>Client-side caching via ETag headers</li>
<li>Admin screen to configure options</li>
<li>Compliance with xhtml and xml standards & many more</li>
</ul>
For more explanation and to leave any suggestions to improve or bugs, please visit on my blog : <a href="http://1uthavi.adadaa.com/ada-wpmu-sitewide-feed-plugin/" rel="nofollow ugc">Ada WPMS Sitewide Feed</a>
<h3>Usage</h3>
Compatible with WP Multi-Network plugin . You can have different blog ID than 1 to trigger the full feed.
<h3>To Do</h3>
<ul>
<li>Comply fully for language translation</li>
<li>Exclude blogs by ID</li>
<li>Enable/disable individual sitewide feeds</li>
<li>Option to include the blog title in the post title shown in the feed</li>
<li>Consider changing the function ada_get_avatar_url() to use the core hook get_avatar(). So it can work with WPMUDEV’s Avatars for Multisite.</li>
<li>Option to exclude by post types</li>
</ul>

Ada WPMS Sitewide Feed

Creates four rss 2.0 feeds showing recent posts, comments, pages, and one combined [posts and pages] from all blogs.  Works both on WPMS, and WPMU

Get All Comments Widget vs Ada WPMS Sitewide Feed