Harry’s departure is a tougher one to rank in terms of classic series departures. There isn’t a story that is dedicated to him leaving the TARDIS but at the same time it might have been much to expect one since he’s only a travelling companion for such a short time. Just six stories and in terms of the narrative of the series, it is just one TARDIS trip (to Nerva and back) so there is never any great expectation that he would continue traveling in the TARDIS once it returned to present day earth. So while his departure certainly isn’t very exciting from a character development point of view, neither is it particularly unsatisfying. There’s a sort of “Yeah, that’s fair enough” feeling about him leaving when and why he does. There is also the sense at the end of Terror of the Zygons that it is not the last time we’re going to see him because he’s a member of UNIT, and indeed that is the case - he’s back on screen a few stories later, albeit not in a “companion” role. So his departure as a companion fits best in the middle of the pack - not one to get terribly excited about or moved by, but under the circumstances not really disappointing either.

One caveat to that - his final appearance in the series is somewhat disappointing - on the one hand it is nice to see him again in The Android Invasion, we only see the real Harry in the final episode so it feels somewhat like a cameo. And I’ve never liked the fact that we don’t see him say a final goodbye in the series to the Doctor and Sarah (although that is the same for all of the UNIT regulars, whom Harry has now been “demoted” to). However, his return appearance doesn’t adversely affect his departure as a companion either (unlike a few other companions that we could - and in some cases already have - mentioned).