This fallthrough is poisonous:

    case PROP_CONSTRAINED_VBR:
      g_warning
          ("constrained-vbr property is deprecated; use bitrate-type instead");
      g_value_set_boolean (value,
          enc->bitrate_type == BITRATE_TYPE_CONSTRAINED_VBR);
      /* fall through */
    case PROP_BITRATE_TYPE:
      g_value_set_enum (value, enc->bitrate_type);
      break;

Results in GLib-GObject-CRITICAL **: g_value_set_enum: assertion 'G_VALUE_HOLDS_ENUM (value)' failed when querying for PROP_CONSTRAINED_VBR (e.g. by gst-inspect or gst_debug_bin_to_dot_data).