One of the major advantages is the handling of out-of-order packets. Packets received out of order over a given channel, as well as packets received out of order across channels (due to latency differences between the channels) are reordered correctly. This makes re-ordered mode better suited at sites with diverse WAN lines, or WAN lines that experience packet reordering from the ISP.
A disadvantage of multilink PPP versus classic mode is there is some extra overhead in the packets due to extra protocol headers. In addition to the GRE protocol (16 bytes), these packets also encapsulate the PPTP (6 bytes) and PPP multilink (4 bytes) protocol headers. And each packet is broken up and sent in fragments across available channels (as opposed to whole MTU packets sent in classic mode). So the additional header overhead is multiplied by the number of channels.