So I watched the Netflix movie of The Old Guard with my wife today, and she enjoyed it more than I did.
Pluses: Charlize Theron and Chiwetel Ejiofor are never not watchable, and the main villain character is a not unentertaining turn by actor Harry Melling, whom everyone knows as Dudley Dursley from the Harry Potter movies. Newcomer Kiki Layne is not bad, if a little lightweight, and those who like action choreography for its own sake will find much good work here. And the theme of the movie -- that people do more good than they know -- is uplifting, if a touch clumsily executed.
Minuses: Several, unfortunately.
First: If the primary plot point of your protagonists is that they're immortal to all normal threats, then you have to come up with something that threatens them in some way for the story to engage us. Immortals fighting mortals is boring by definition because you know your heroes can't possibly lose, which is exactly why Highlander was exciting where Guard's fights are just dull (unless you're a fight-choreography junkie). If your heroes can't be personally hurt, then threaten something they care enough about that that's worse, which would have been easier had these characters been given enough personality to have such people or things in their life.
Second: If the villain doesn't start out with a personal connection to the heroes, he'd better acquire one damn quick to make the story interesting. Melling does the best he can with his villain role but his problem is that he really doesn't care anything about who our immortal crew are, or have anything personal against them beyond their natural reluctance to be his guinea pigs for reverse-engineering immortality. As a result the fights are basically exercises in choreography and nothing more.
Third: If the audience can think of a simple and more effective tactic against the heroes than any the villain uses, requiring only what the villain already knows, then the villain is a dull villain. If your immortals can cut down entire squadrons with their blades while ignoring firearms, then do not get near them -- stay out of range, use tear gas, Tasers and tranq darts.
Fourth: The great thing about the immortals of Highlander was that, even though we never knew where they truly came from or why they had to fight for the Prize (I am ignoring HIGHLANDER II here, but doesn't everyone?), the simple fact that there was a Prize and a fight they couldn't escape put a forward motion on all their life-arcs, no matter what else they did. The immortals of Old Guard don't seem to have any reason for their existence, and no reason for the only possible way they appear to be able to die: Also, the immortals we see on screen are, with one or two exceptions, all there ever are or have been; the story deliberately cuts off any hint of a larger, mysterious world.
Fifth: I loathe the SJ advocacy movement with a passion for putting these thought patterns in my brain, but they're there now, and I cannot help notice: Of our five immortals, the one who turns out to be a traitor to the group is, predictably, the lone straight white guy of the bunch, and the gay couple who've been lovers for a thousand years also happen to have been on opposite sides in the Crusades. My good faith that such choices aren't a not-so-subtle "eff you" to certain viewers has, sadly, declined in recent years.
Final score: 1.5/5 solely on visual style and good acting jobs from Charlize and Chiwetel. If you really want a good story about immortals go back and watch your Highlander DVD.