I've been doing this for decades, starting with petrol engines and then diesels. I have a 2008 CRD to which I added a Chip Tuning programmable diesel chip, what was then the Chip It and is now the Dominator diesel chip. There will be a number of views on this issue, with some insisting that ECU reflashing is the only way to go and others the piggy back chip. I'm in the latter camp, especially when it comes to the Dominator chip, as it allows vehicle specific tuning to be done by the owner if they wish. I wrote about my experiences here: https://australianimage.com.au/nissan-gu-patrol-diesel-chip/.
With ECU flashing, unless you can take your vehicle to the tuner, involves canned tunes that may not bring out the best in your vehicle, though this can also apply to chips. Secondly, if you can't take your vehicle to a tuner, you have to remove the ECU and send it in. If the tune is not right, rinse and repeat and there's no guarantee that it'll ever be right. You also have to be careful who you send it to, as not all reflashers are reportedly reputable.
The Dominator, in this instance, allows you to select from (I believe) 100 tunes now, so the chances of getting one that suits your vehicle is fairly good. But if you want optimum, it can also be specifically tuned for your vehicle. I tuned one owner’s vehicle this way (I have the dealer software) and the owner went from ready to sell his Patrol to keeping it for good. You need to understand that no two engines are identical and a tune that brings out the best in one vehicle, will rarely will do the same with another.
A chip will make a CRD Patrol sing, blow no smoke, get great fuel economy and cause no issues. You won’t even think about a 4.2 if you have a chip tuned properly. Whatever you do, you should in the first instance get a 3" exhaust, as well as a boost and EGT gauge, the latter are essential for proper tuning.