The main problem for me was that I was not able to complete a train ticket purchase with a US credit card. I tried 3 cards and the transaction always fails as soon as Pay Now is clicked. It doesn’t say a thing but no confirmation shows nor email is received. I tried the website, too. It always fails there, too, but, at least, it lets you know that it was rejected by the credit card company. This is while you wait for a text message with a passcode from your credit card company. The rejection happens before you have a chance to enter the passcode.
Also, there is a reserved seat option in ÖBBTickets , but you cannot select the actual seat. The website, however, does let you select the seat. So if you are in Austria use the website, if you can. For me, I had to ask a favor from an Austrian acquaintance, who purchased the tickets for me.