Saga markets itself as an all-inclusive, luxury cruise line. However, with the introduction of their new ships, things slipped a bit, especially post-pandemic. Generally, the transport TO the ship is excellent, but it has been noticed (and commented on by many) that the transport home after the cruise can be sub-standard. It also seems that on occasions, Saga have resorted to using Uber, which is not necessarily the same standard of driver or vehicle as one expects when they have previously used Titan Mercedes MPVs or similar.
Also, their "included" insurance has so many exclusions - it is much more likely than one can be refused that cover - they prefer to suggest you try the "paid" insurance policies from their own insurance arm (and this can be more expensive than travel ins policies from other companies).
Food is always controversial, but even pre-pandemic, it was noticeable that portions became smaller, and the choices less, or more limited. Menus got repeated more often too. The choice of included drinks is basically "house" wines and spirits - branded drinks have to be paid for.
They say you get included excursions, but these often are only panoramic ones. If you actually want to explore a bit more 'in-depth', you have to pay for it, or book your own.
I think Saga now base their expensive fares on the fact that every cabin is an outside cabin with a balcony. But it is very noticeable that their range of cruise itineraries have become very limited and repetitive. I think before you make the "leap" from P&O (or indeed any other line) to Saga, you need to compare prices on a like-for-like basis (meaning similar standard of cabin and similar itinerary.