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Exchange historically had a tendency to mangle emails sent through it (whitespace changes, line wrap, etc), which is obviously bad news for patchmails. I dunno if it's any better these days.

git-send-email also has some quirks for Outlook which have been recently merged.

And I think you may underestimate the power of stgit. You can manage thousands of patches concurrently, no problem. If you're a maintainer getting patches from loads of people all the time, this is valuable. stgit has it's origins in quilt, which in turn has its origins in Andrew Morton's patch scripts[1], and I know for a fact that Andrew Morton actually managed thousands of patches at a time for years in his work on the linux kernel, because I once sent him a patch against those scripts, and he complained it was slow because I used an O(n^2) algorithm, which worked fine with a handful of patches, and I asked him how many patches he had, and he told me a number that was multiple thousands, so this isn't a hypothetical example.

[1] https://lwn.net/Articles/13518/