It's low level, so by low mid+ levels a cleric can cast it as needed;
It's ranged so the cleric doesn't have to move (or not move far) to get to the target;
It doesn't heal much but it heals enough to prop up a PC to do what they need to do AND it nulls/resets any death saves;
It's a bonus action meaning the cleric can do other stuff, sure more limited stuff, but it's one of the few call backs to the 4e style heal AND do stuff cleric.
The first is a useful attribute, but irrelevant to whether the amount of healing provided is particularly effective
other than as "whack-a-mole"/"pop-up" healing.
The second is, likewise, useful but not super relevant, other than for the ease of use with w-a-m/pop-up healing.
The third, however, is exactly what is being said. 6-9 HP is diddly-squat for a high-level character, even if they have a low HD and low positive CON, e.g. a 12th level (non-Dragon) Sorcerer with +1 Con has 7+5*11 = 62 HP. Even a maximum roll on
healing word gives a grand total of 14% of your maximum HP; and God forbid you actually play a high-Con class with larger HD! If you take a single hit from many creatures, even very low-level ones, you're going to go right back down into the dirt.
The fourth is, of course, another benefit, but not really relevant to whether the healing itself is
a serious amount of healing.
Healing word is a very useful spell for specific circumstances and contexts. It is valuable to have (one of several reasons why a 1-level Cleric dip is very valuable), but that doesn't mean it is a
powerful healing spell. It is, in fact, the
weakest proper healing spell in the game. It just has, as you've noted, enough special features that its hilariously tiny healing (1d4+mod!) is
forgivable.
That doesn't make that hilariously tiny amount of healing suddenly not hilariously tiny.
As I said earlier in the thread: It is 100% correct that healing needs to be a limited resource in
some sense in order to preserve tension in combat. Making it weak (as in, very small
amount) but plentiful (as in, you can cast it many times a day) incentivizes tactics that many people strongly dislike, and it incentivizes those tactics
specifically because it is weak healing, best used when it revives an ally that is already downed but not out. Had 5e designed its healing in such a way that it could instead be
strong (as in, a comparatively large amount) but
rare (as in, a limit on casts per combat), it would instead have encouraged tactical caution and taking seriously the possibility that an ally could
die if allowed to hover at very low health for long.