This topic is interesting and useful, thank you.
My case is different. I am not a programmer, a computer or a network specialist, I have been working for 26 years with graphic design and prepress of books, magazines, advertising materials etc.
I have a good knowledge of HTML and CSS, some basic notions of PHP and I do not use ready-made themes and layouts but I develop them myself for both Drupal and Backdrop.
I'm self-taught in everything related to site development. The last 10-12 years I have done over 120 sites, currently I maintain over 60, of them 10-15 with Backdrop, one with Wordpress most of the others with Drupal 7 and only a few with Drupal 6. I will definitely not use Drupal 8.
I am not able to install and support my own web server (also to work with Git, Drush, Composer etc) and I use shared hosting with a reseller account, which allows me to create separate hosting accounts for any site I am developing or supporting.
That's why I don't use multisite, even more - my hosting provider disagrees in one hosting account to have multiple domains unless they have the same owner/company.
Another reason is the different life cycle of each site and the access of users to it.
I have clients with almost full access to the site and hosting accounts, who can even install modules and others where I am the only one user in the site.
I don't think it is good idea to give my clients access do hosting account with many other sites that are not theirs.
It happens to transfer the site along with its hosting account to another admin. Or it happens to me to be transferred site with a hosting account, developed by another developer.
And I have sites in different hosting providers and often I am not the owner of the hosting account, but only have access to it temporary.
Some old sites that are not updated with new content I often convert to static HTML sites and save Drupal or BD installation on a local server or backup archive for possible future work and development. This also eliminates the need to update them for security updates.
For all these reasons I do not use a multisites but separate hosting account for each site.
And when I need to update the security release I really do have a lot of work to update each site separately (I work via FTP and cPanel with hosting accounts).
But it is not so scary, I update one site for 10-15 minutes- including backing up the site before and after the update.
During the last Drupal (March 2018) security release, I had several Drupal sites hacked before I could manage to upgrade them. Nothing serious, just an unknown new user "Alex" registered on the site 48 years ago!
I recovered them from backups.
To protect against such attacks on sites that I have not managed to update on time I use (in Drupal) several useful modules that I would like to see in Backdrop too (some already have Backdrop versions):
backup_migrate - for a backup of the site on schedule, keeping the last few months backups of DB and files
md5check - tells me when core or modules file is changed on cron run
rules - notify me when there is a new user's registration not done by me; have plans to make rules notify me of md5check records in system log
access_filter - to grant access to important admin pages only to my IP address
secure_permissions - hides permissions and role admin pages
userone - hides User 1 and block access to its profile
restrict_by_ip - to prevent access to all admin pages of the site that is not yet updated with security release
When it is possible I deactivate or delete Administrator role at all and block User 1 manually editing user table of database via phpMyadmin.
I'm not sure how much these measures are really secure, but I guess it's better with them than without them.