One disadvantage to pull requests (aka merge requests) is lots and lots of merge commits.
It's not the worst thing, but it does clutter the commit logs, and make for lots of unnecessary non-sequential history.
A - C - E - G / / / B D F
What I think would be better is to cherry-pick the commits.
A - B' - D' - F'
Of course I can do that myself, but one of the advantages in a pull request system like Github, Bitbucket, Stash, or Gerrit is the ability to do this through the interface, immediately following a review of the changes. And doing that manually wouldn't show the pull request as accepted in the UI.
Why don't pull request tools offer "cherry-pick requests"?
I understand the advantages of merges and non-sequential history in the general case. (Otherwise, I'd be using SVN.) I am specifically taking about one or two commit changes from transient branches. (Many tools even help you delete the branch right after merging.) A one-commit change is probably the most common case for pull requests.