My advice.... stay with the architecture/style of the home and you will not have a big problem. Most of a house can look newish with paint and maybe some floor refinishing if it is bad...My next project should be a full makeover of the 30 year old bathroom. The problem is if I go luxury it makes the rest of the house look shabby by contrast. Plus I don't want to spend the price of a new car on it.
I once saw a gilt and marble bathroom in a virtually untouched 1940 house. It was like stepping out of Taj Mahal and into a concrete plant.
Long version....
While there will always be contrast between new and old (shiny and dull) when a space is updated IMO most of the bad contrast is because people don't stay true to the style/architecture of the rest of the house--specially for kitchens and bathrooms. So the updated space seems out of place, or maybe the rest of the house now seems out of place....
As an example my 1940s house is Minimal Traditional, which is pretty neutral overall. BUT, if I put in some modernist kitchen/bathroom it will stand out like a sore thumb, Same if I go Victorian etc. If I stick to the architecture/style of the home the space will fit. In my example I can pull in some minor elements of just before or after eras as long as I tone them down and it will still look good (for example a touch of craftsman, tudor revival, really toned down art deco or MCM). My kitchen reno has been delayed by other projects but is coming, no way I am doing a Minimal Traditional kitchen which was generally bold painted plywood cabinets but I will pull in elements that work with the goal of an end result that looks new but also looks like it could have always been there.
The problem in general is most people see something in a showroom or on HGTV, like that in a vacuum but it just does not work in their home... combined with architecture not being a general knowledge to even know what they are starting with.