Okay, who wants some bargain-basement soapboxing on this one? ;)
Let's start with the good part: David Warner is pretty amazing. Given the long stretches of film in which there is little to no dialogue, he has plenty of opportunities to do some really great work with just a glance or the minutest gesture and he manages to constantly be watchable, even when the plot threads around him drag on or feel fleeting and under-developed. The scenes with Alice St. Clair as Zooey are handily the best in the film (and there really should be more of them; that fresh, unclouded perspective between the young person who doesn't really even know much about his work so she just sees him for who he is, not for who she thinks he is or for who he could have been... I feel like the movie needs more of that.)
I'd say if anything, narratively the movie suffers from being too committed to the concept of its protagonist's failing health and mental state. And this is me straight-up giving the benefit of the doubt to the Sharff brothers and assuming that this aspect of the movie is intentional... and not just the result of thinly plotted, overly dream-like storytelling. Allow me to explain (and what do you know, the best example involves the portion of the film that Jamie is in? Convenient! ;) ):
The first clue, if you're looking for it (I wasn't; I went back and looked after I'd finished the film) is in Phoebe's obituary in the paper. It lists her birthday as 1938, but notes she married Paul in 1971; hence, she was 33 at the time. There is no plausible way Bonnie Wright looks remotely close to 33, in the film or otherwise. (The same could be said of Eugene Simon.) So in the flashbacks, we have the two of them represented very young as they likely were when they were in love... but this isn't necessarily representative of the Phoebe who ultimately married Jamie's Paul. It's the way Eugene perpetually remembers her, regardless of what part of her life we're getting a glimpse at.
Meanwhile, the scene by the river - reeeeally implausibly - gives us Phoebe having three key conversations that dictate her future in rapid succession: Rachel, intimating that Eugene is a poor marriage choice; Eugene himself, and the moment in which she basically breaks it off with him without strictly saying so; and Paul, being - and this is important - both clearly sexually appealing and broadly demonstrating a creepy, predatory stance toward Phoebe. There's nothing either before, during, or after the Eugene/Phoebe scene in the middle that definitely indicates he was lurking nearby (In the bushes?!) or privy to either the Rachel or the Paul conversations. So are those two bookending conversations the way Eugene chooses to understand things?: i.e. one of her friends put her off the idea of marrying him, and then a brash, brawny braggart like Paul swooped in and stole her away? (Though he apparently sees her as a sexual object and not as a woman?.... But, what do we know of Eugene's relationship with her that demonstrates something clearly to the contrary, other than their having been childhood playfriends? Literally, that is the only thing we have to go on with regard to her character and why he is so besotted with her. NOT ENOUGH.)