I agree with those pointing out the difference between illusions and phantasms. They are separate things, though are all "illusion magic". Like
Cause Fear, Friends or
Charm Person is a "charm" or "enchantment" and
Suggestion,
Command or
Hold Person might be classified as a
compulsion are both "enchantment magic." They [phantasm or compulsion] are a next/second "grade" of those magical disciplines.
Phantasms will react to interactions. They are "real" enough to the target to deal "illusory damage" that cause the target to act accordingly, including "passing out" when they believe themselves to be dead, or in the case of phatansamal killer
feel real enough to instill a heart attack/stroke/brain death/psychic attack/however you fluff the "killing" side of the spell.
Phantasmal magic, not illusions, are the bread and butter of the illusionist class [non-phantasmal illusions are, I suppose, the jam...a nice colorful additional, though largely unnecessary, layer on top.

]. And, also as noted by several people, it
does rely on the creativity of the player, really, moreso than the power of the spells, to show its real colors. This [as a separate conversation: "Levels of Player Dependency of Different Classes"] could be considered a bug of the class, compared to other magely traditions...I personally consider it a feature, but YMMV.
As that may be, it doesn't seem correct or fair to claim the illusionist class is "underpowered", or any more underpowered than it ever was in 1-2e. The tradition special powers are certainly not the "wow" of some of the other traditions...but then, that's the difference of the traditions. They should not expected to match [power/flavor/usefulness] 1:1 across all 8 traditions...and really,
can not be if the design goal/intention was to make different tradition mages
feel/play differently vs. a simple flavor/fluff trappings with no mechanical impact.
I'll also chime in to agree with the people that say that no illusionist worth their salt would ever
tell anyone they are an illusionist. And certainly wouldn't make it clear to enemies. They are a WIZARD (which they actually really are)! Seems to me, with 5e's capacity for them to continue to cast things from any other school, makes this more true than it's ever been (though granted having largely skipped 3 & 4e, I can't really speak to how the illusion specialist mage played in those editions).
So, yeah until I see one in play...and I suppose played poorly...I can't really agree with the thread's premise that the illusionist
wizard is or "seems" underpowered.