Following are incompatibilities between PXC 5.5.33 (and higher) and older versions:
wsrep_sst_donor now supports comma separated list of nodes as a consequence of bug #1129512. This only affects if you use this option as a list. This is not compatible with nodes running older PXC, hence entire cluster will be affected as far as SST is concerned, since donor nodes won’t recognise the request from joiner nodes if former are not upgraded. Minimal workaround is to upgrade Galera package or to not use this new feature (wsrep_sst_donor with single node can still be used). However, upgrading the full PXC is strongly recommended, however, just upgrading PXC galera package will do for this.