This isn't a complaint about the overall migration of questions from EL&U. I think taken as a whole, the migration path is a positive thing. I have started to see a few real stinkers getting migrated though, so I want to do a little level setting.
While the proposed duplicate is closely related, this is a discussion and not something that has an answer. I'm not sure what purpose would be served by closing this as a duplicate of a far older question. Folks come and go from both sites and I think it is worthwhile to revisit the migration path every so often.
There is new information here that isn't in the previous post, and my message isn't "don't migrate garbage". I think that the folks that are migrating these questions are trying to help the author. I am trying to explain why migrating an off-topic question might not be the best way to do that.
When I wrote this post, I had just rejected migration of a question with this as entire content of that question:
is " Which cities did you go to?" a correct question?
Proof-reading is off-topic on ELL, just like EL&U. Migrating this question doesn't help the author of it. It just makes things more confusing because we can't put it on hold while we help the author bring it on-topic. When you see learners, feel free to send them our way with a comment, but please don't migrate one line "is this correct?" questions. They're problematic for two reasons - there isn't enough detail to understand why the author is unsure about the sentence's correctness, and they're of limited usefulness for future visitors.
The migration makes the ELL "on-boarding" process a little tricky because users don't automatically get an ELL account created. Often they end up creating a second account and then they get confused because they can't edit their question or comment on anything. It's not a good experience. If the question is OK to start with, most of the time the "on-boarding" goes a lot more smoothly.
If you're curious about which migrated questions are well-received and which aren't, here is a SEDE query that let's you examine migrated in a score range starting from a particular date. The default date of 09/22/2015 is when the migration path opened between EL&U and ELL (before that questions were migrated by moderators). Keep in mind that many of the really poor migrations won't show up in that query, because they've been rejected and roomba'ed by now.
This migrated question, for example, was well-received by the ELL community:
Does 'yield' still agree with 'the subject' in 'measures can be undertaken that yield/s positive outcomes?
It has a good amount of detail and is about something of interest to many learners. On ELL, enough context/explanation of what is confusing can make up for a lack of research. Ideally, questions would have both context and some demonstrated attempt at research (i.e. not just the statement "I googled and couldn't find anything"), but we can get by if they have one or the other. If a question has neither, please don't migrate it.
We've also started a community wiki answer on ELL's meta with links to a few questions that we think are good examples of questions with enough detail with the hope that illustrations of "good detail" will be more informative than trying to come up with a documented comprehensive guideline.