I tutor children from a lot of schools, and many use a variety of these systems - they're all pretty similar.
They are helpful for those who struggle with getting the homework written down, legibly and in time, and with those who have real organisation problems and manage to lose all the sheets so that they can't actually do the work, and start falling behind.
But for children without those difficulties, it has resulted in them adopting a less responsible, less organised approach. There's always someone or something to blame. Even if the teacher has told them homework in class, if it's not on there, they can get out of doing it, or they didn't listen the first time. they don't listen properly if the teacher tells them about tests or other useful information, as 'it will be on edmodo' or whatever. it's just taken the responsibility away from them, as well as the ability to organise themselves. They don't see longer term than what is immediately due that week, in the way the do with a paper diary that they can flick through to see when half term is (when they know exams are often just before it), what tests are coming up in two weeks, when a project is due, all that sort of thing. They also can't write in things that are commitments for themselves, like play rehearsals or school trips, so that they can organise the homework around it and plan ahead.
All of these things are perfectly possible with an online system, when there are motivated students, supportive parents, supportive schools etc., but my experience is that for the average child, they often don't happen. They stay reliant on someone else to organise the homework and tell them when to revise. I teach university level students as well, and I can see that some of them have also never developed these skills, and wonder if there is an argument for going back to paper diaries where you can see the weeks ahead more easily and keep all the commitments together in one place.
The ones used most often here seem to be google classroom, edmodo, show my homework, epraise.
There is also a problem with students having to remember to write down all the various log-ins and codes that they need to add the right classes to the system (if not done by the school), and to keep all the passwords and codes separate from the ones they need for the various online maths, programmes, etc. I know some who have logged in permanently on their phone, but lost the password since, and if that ever is inaccessible, they can't access the homework at all, since we've tried on other devices.