You cannot really compare them. Mastodon is similar to Twitter and nothing more, it’s a microblogging service. Lemmy is like Reddit or the Usenet from back in the days with a nice threaded view. Many people on Reddit never used Twitter and vice versa.
This new fancy stuff is all part of the Fediverse, but that‘s a bit misleading. You can use one part and completely ignore all the other ones.
A very little reminder that CrowdStrike for Linux (yes, the same falcon that crashed on Friday) caused kernel panics at least on Debian after a broken update in April. So the issue is not the operating system but the vendor not having a proper quality control (which is no big surprise, the current CEO was CTO at McAfee when they rolled out a broken update in 2010 which caused massive outages worldwide)