I've not replaced my wall oven, but it seems narrower than other ovens I've used, and to replace it WILL require new cabinets.
I did replace my cooktop (I went from electric to gas) and I did have a problem with the size. The cooktops do have standard sizes, BUT the cut-out required is NOT standard. I had to enlarge the cut-out to accommodate my new cooktop. I also need to shorten one of the drawers underneath, as it will hit the gas connection if inserted.
When did work on my electric cooktop I found that it was handy to pull it out to fool with it.
If you have a range (oven + cooktop in one unit), one can presumably disconnect it, and drag the whole thing out into the middle of the kitchen when needed for servicing. It's a bit of a pain, but not horrible to do, especially if you're all electric.
If you are planning to put the oven under the cooktop (as shown in your first photo), then I'm unclear that you get much benefit (service-wise) from the built-ins. You can't easily service either component because the other one is kind of in the way, so you'll end up having to go in sideways to unbolt them and pull out. And pulling out a range is relatively straightforward - it's free-standing anyway, while pulling out a wall oven is going to be a pain.
One thing I did notice is that ranges seem to be SIGNIFICANTLY cheaper than buying the components separately. A cheap range can be had for $400, which is what you'll spend on just the cooktop if you go independent parts. I rather assume that parts cost will trend with cost of the appliance, so I'd assume that cost for repairing built-ins is higher.