Importance of uniformity of hardware for Galera cluster
You basically answered your question yourself: the main problem is uniformity of response time- you cluster, as a whole, will be as fast as the slowest of your nodes (due to the roundtrip). This may affect not only your hardware choice (for example, all nodes having SSDs except one will be a bad choice) your network topology too- a remote server will slow down nodes on the same data center. Also, you do not want one of the nodes to run out of space, which may take some time to be desynced.


  the nodes use the same disks, the same amount of RAM and similar CPUs, shouldn't that be good enough?


Yes, "it should be enough", now the question is if it actually is in practice. Makes some tests on latency, specially under load (mysqlslap/sysbench). You never know when a small detail can be a big difference. It is also easier to manage uniform hardware and software (for replacements/fixes and for comparing performance). But you are mostly right.