I cannot comment on their reliability as I disabled them quite a while ago due to excessively popular posts too many alerts. I know at least some of the other staff have also suppressed "like" alerts for similar reasons.
That said I've added a few likes for testing purposes. I wonder if it has something to do with multiple likes on the same post and/or thread? For example, if I'm subscribed to a thread I'll see "Kyriakos replied to the thread "Like alerts sometimes don't work". There may be more posts after this" as an alert, but won't get subsequent alerts should someone else also reply to that thread. Maybe it's only alerting once per post (until you check), or for one post in a thread if multiple posts in that thread received likes? Theoretically, you should have received two alerts from my likes in this thread (and Valka received one).
If you can spot the pattern, it can likely be shared with XenForo as a bug report.