Multipath routing - Stuff to know

return path may not match forward path
asymmetric routing
may confuse firewalls in between

routing over different interfaces/networks is tricky
outgoing connections choose one interface as source
but the packet is routed via the other interface
result: traffic may get blocked by antispoof or URPF checks
forwarded traffic is not affected

link loss is not (yet) detected
only a part of all connections will work.
ifstated(8) plus a shell script may help

incoming traffic can not be load-balanced