LINQ encourages the belief that set operations are free, such that you no longer have to concern yourself with concepts like memoization or appropriate structures. This has nothing to do with functional programming. Literally at all.

Basically, you're saying it's too easy to accidentally perform a computation twice by iterating through a sequence twice. Recurse on that problem and you get an exponential blowup.

That is actually a problem mostly unique to LINQ and it is really important that a programmer understand the deferred semantics. Some tools, e.g. ReSharper, will detect multiple enumerations and warn you about it. Judicious use of ToArray/ToList solves most issues with defer-splosion.

> "it falls apart in real-world persistent code with considered algorithms and storage"

> "it makes it so easy to do [grouping / sorting / etc] that developers start to resort to that as a catch-all magical solution that is costless because how much could a line or two of code cost"

With hindsight I know your issue is with LINQ's deferred non-memoized execution. It's really easy to enumerate a sequence twice, but every place you do that might add a factor of 2 to the runtime and so there's a danger of exponential blowup that must be avoided. But that's not what I see when I read the above.

The above reads as complaints about easy composability and worries about code monkeys failing to consider what the code they're writing will do. Those are standard "functional / high-level programming is bad" complaints. With hindsight I know that's not what you meant, but that's what it reads like.