While boards such as these can be helpful, sometimes you have to take what is being said with a pinch of salt. Everyone has different likes and dislikes. One of the big topics on changes is the no night time turn down. I don't think you would you get this if you stayed in a hotel at Spain, so straight away something not to worry about. Food is another topic which is personal to people and there is plenty of different places onboard to eat to please most people. Reports of food not being very nice can just be down to a persons own taste.
Are the likes of there being no sweets in the cabin or the fact that they no longer provide small bottles of shower gel going to affect your holiday. Most places are now providing shampoo etc fixed in large bottles to the wall. While people will argue it's a cutback, it's a move towards sustainability and won't effect the enjoyment of your holiday.
While a lot of people who have been cruising for many years have seen lots of small changes, the same thing is happening on-land. Also they are still taking multiple cruises a year with P&O, so they must be doing something to make them return.
We have really enjoyed the P&O tours we have taken but some people wouldn't entertain them. Again it's personal choice. Some people prefer suites, some balconies and some inside cabins. They might say they would never go in another type of cabin. That doesn't mean there's anything wrong with them, it's just down again to personal taste or affordability.
Go and enjoy your holiday with an open mind not worrying about what other people think. Don't be put off by negative posts as people tend to post more about something they haven't enjoyed than something they have. The crew are all lovely and Iona is a new ship. From the photos and videos I have seen she is lovely. If you don't like one part of her just go to another place.
You will have a fab time and be wanting to return again. Just don't go thinking that you won't enjoy it, you will.