I recall when Spirit of Britain arrived from Aker Finnyards, she had a few teething issues. Even during the sea-trials Aker had to do some fine tuning, (Tho I don't recall Spirit of France having any issue).
I agree, its a PR and logistical screw-up, but even highly experienced yards like Aker Finnyards, who built a whole string of vessels (including Ulysses), can have issues.
Likewise, van-der-Giessen de Noord, despite building a vast number of very successful ferries - Konningin Beatrix (1986), Norbank (1993), Norbay (1994), Isle of Innisfree (1995), Isle of Innishmore (1996) etc, were late delivering Mont St Michel to Brittany Ferries.
I am not sure the pricing structure, but I was quoted €307 return for two people, from Dublin Port to Cherbourg on WB Yeats for around mid September. The €150 voucher or the option to sail on an older ship is quite dissapointing. Though to be fair to Irish Ferries, Brittany Ferries are being quite brazen charging around €800 for a return with a car, 2 people, and a cabin, on a "very basic - no frills" Visentini - a vessel comparable to Epsilon and Stena Horizon. Even at the very end of the season Brittany Ferries are pricing €600+ and that's not including restaurant etc. I understand there is more fuel consumption on a sailing to Spain vs one to France, but I don't recall P&O charging anywhere near that to sail on the much better appointed (and thirstier) Pride of Bilbao (Portsmouth to Bilbao).