When using avfvideosrc on iOS I observe a crash with the following backtrace (edited slightly to remove sensitive information):

(lldb) thread backtrace
* thread #35: tid = 0xd3ab1, 0x002ebaec`gst_core_media_buffer_priv_new(ctx=0x1b0350c0, sample_buf=0x00000000) + 32 at coremediabuffer.c:77, stop reason = EXC_BAD_ACCESS (code=1, address=0x50)
 * frame #0: 0x002ebaec`gst_core_media_buffer_priv_new(ctx=0x1b0350c0, sample_buf=0x00000000) + 32 at coremediabuffer.c:77
   frame #1: 0x002ffa88`-[GstAVFVideoSrcImpl create:](self=0x15e14320, _cmd=0x35dfb0d2, buf=0x09cf4d68) + 948 at avfvideosrc.m:814
   frame #2: 0x00300b8a`gst_avf_video_src_create(pushsrc=0x16951600, buf=0x09cf4d68) + 126 at avfvideosrc.m:1203
   frame #3: 0x0023aa9a`gst_push_src_create(bsrc=0x16951600, offset=18446744073709551615, length=4096, ret=0x09cf4d68) + 78 at gstpushsrc.c:130
   frame #4: 0x002295ce`gst_base_src_get_range(src=0x16951600, offset=18446744073709551615, length=4096, buf=0x09cf4ee4) + 530 at gstbasesrc.c:2445
   frame #5: 0x00228708`gst_base_src_loop(pad=0x16236138) + 784 at gstbasesrc.c:2721
   frame #6: 0x0017310c`gst_task_func(task=0x16a43ac8) + 632 at gsttask.c:317
   frame #7: 0x00173a52`default_func(tdata=0x16190e28, pool=0x16155038) + 46 at gsttaskpool.c:68
   frame #8: 0x008d47b0`g_thread_pool_thread_proxy + 84
   frame #9: 0x008d3d3a`g_thread_proxy + 118
   frame #10: 0x3ae3b958 libsystem_pthread.dylib`_pthread_body + 140
   frame #11: 0x3ae3b8ca libsystem_pthread.dylib`_pthread_start + 102

I am sure that avfvideosrc used to work on iOS and it does work on OS X.