The iOS version sometimes won't connect when the Android/Windows/Linux versions will. It has to do with the iOS version using different ports.
Here on the local repeater we had to work with the ISP to get certain ports unblocked before iOS users could connect, but I wasn't directly involved so I don't know the particulars.
i have used echolink for android AND the Iphone as i got an Iphone earlier this summer. both look and work the same from what i can tell. no problems with either. i dont use it that much, but have not noticed anything.
I had it on my Galaxy Note 5 running Android 6.0 and 7.0 and it worked fine as other people have mentioned. If I recall correctly, it takes a day or so for the system admins to approve your call sign before you can use the app. I ended up deleting it because I simply wasn't using it and I do not like having a million apps on my phone but if I had a need for it I wouldn't hesitate to download it again.