We have a Pyronix system, as we have had in our last couple of houses too.
All wireless now, so straightforward to install. Not issues with battery life, or false alarms. It was installed by a local alarm company, but I don't see why you couldn't do a self install.
What we like is that they now have an app that you register the system with and you can set and unset the system as well as check its status via the app.They do charge £36 a year for the subscription.
The one issue we have is that maybe once every few months there seems to be a problem with they system for a few hours and you cannot set it remotely. You can still set it via the the panel.
You can use their app with CCTV also, but we have a separate app for that at the moment.
Would the sills be in a spot where people would likely put something on them? If they are just painted in mat emulsion and people start to put vases and tea cups there it will stain. If not then I don't see why not plasterboard, we have some plasterboard alcoves which are basically the same idea.
I have one of those, but alas no detector thingy. It works well inside but is very hard to see outside over more than a couple of metres until it gets dark. Then next door can see it and think an alien invasion is happening. Mine appears to be sufficiently accurate for everything I've asked of it - most difficult was a height comparison over 15m distance.
I used a more expensive red rotary laser with a detector-on-a-stick for getting the level of my blockwork because it was conveniently available.
"All the interfaces" and "Shield pluggable" are the main reasons. I prefer, at least for simple projects to keep things properly simple. I don't want to build in too many complex toolchains, opportunites for obsolescence or have to worry about somebody else's PCB design and track routing. On the whole, a shield based design drives me towards a more convoluted power supply and doesn't save all that many components, although the easy USB and possibly TCP interfaces would be nice to have. Processor speed isn't an issue because my design needs nothing to happen particularly quickly by modern standards.
I can still get away with those optivisor things but it's getting harder.