Hello
I have been assembling alarm systems for 22 years (including for about 19 years perimeter systems with reed sensors recessed in frames and inertial detectors).
Never has any producer or seller of windows had any objections to the installation of sensors by us, let alone withdraw the warranty due to the installation of sensors outside the factory.
At the beginning, maybe a word about why installing reed switches at all.
Well, in order to:
1. Create an effective alarm system that quickly detects an intrusion attempt
(reed switches in combination with inertial detectors).
2. Be able to move freely around the house with an armed alarm system.
3. Being able to sleep with a tilted window secured by an alarm system
4. Be able to detect an open or tilted window or the position of the handle when leaving the house (depending on the type of sensor and mounting method).
5. Be able to leave pets at home.
It is not true that a good alarm system will replace mechanical protection.
You should not give up on fittings and glass with increased burglary resistance.
Only a combination of mechanical security (minimum WK1 fittings and minimum P2 glass) in combination with a full perimeter system (reed switches + inertial detectors) + motion detectors in the main points of the house (3 - 4 pcs.) Is an effective protection of the house. The thief will trigger an alarm at the first impact on a window or glass, try to hammer a crowbar or otherwise intervene in a window or door, and stronger mechanical protection (WK1 - WK3 hardware and P2 - P4 panes) will keep him outside. He won't have time to wrestle with the window any more, because an emergency patrol is on its way.
Nor is it true that reed sensors are easy to bypass.
First, you need to know how many sensors are in the window and where they are installed.
Second, you need a very large and strong magnet.
Thirdly, the reed switch is a sensor that complements the perimeter system - the main element of protection is an inertial detector.
Fourth, there are many types of such sensors - including fraudulent detection and RFID sensors - that cannot be bypassed.
Regarding the installation of reed switches by the window manufacturer - Customers threatened with the loss of the warranty in the case of installation of reed switches by alarm system installers, sometimes decide to install the sensors by the manufacturer. They pay a lot of money for this service, and the results are meager.
Here are some examples:
Badly installed sensor - not only is it not working, but it has also been destroyed by the magnetic roller of the fitting, which tore the plastic cover of the sensor and damaged the inside of the detector.

Here is another example of "expert" sensor assembly by the factory.
I think they used a penknife to pick holes for the sensors. Too small holes and the lack of chamfering prevented the sensors from being fully inserted, so they protrude slightly. Questionable aesthetics.
In addition, the sensors were hammered in and some of them were damaged.

Of course, alarm system installers also sometimes do not have the knowledge, appropriate tools and experience, so they bother installing the sensors.
World record here:
The client did not accept our offer because it was probably too expensive. Took another installer. This one had no idea how to install reed switches in the windows.
He glued ordinary, top reed switches to the bottom strips collecting water dripping down the window and made holes for wires in them - probably there will be no water leaking inside under the windows and window sills ...
This installer broke down after "mounting" the reed switches. The client later called us to finish, but due to the mass of other flowers, we did not take up the topic.

Here are pathetic attempts to match the magnet to the sensor - one too little, it gave two

In addition, drilling on the joint of the window frame (and multiple times) - with such an installation, the window manufacturer will probably automatically withdraw the warranty for the window!

Here, the top reed switch on the roof window - it seems to work, but the aesthetics leaves much to be desired ...

Here is another "professional" assembly with a magnet visible from the outside of the house!

In my experience, companies producing / selling windows install any kind of sensors, and after installation, they do not even check whether the installed sensor is working properly. People in production do not know at all how such a sensor works and, as a rule, are not able to set it up and test it. They do it without knowing it.
It is completely independent of the window manufacturer, price or type of sensor. Recently, one of the largest Polish companies producing windows and doors has installed the highest model of windows (oak, triple-pane with P4 anti-burglary glass + WK3 anti-burglary fittings and top-class opening sensors - RFID) for one of its clients. Windows for about 100 thousand. (medium-sized house). Several sensors were installed incorrectly. Their service came several times and they did not know what was going on and what to do with it. Ba! Even the chief technologist of such a large company did not know! Why go for something that you don't know?
The RFID sensors alone cost several thousand zlotys! Failure!
Several of our clients decided to install system reed sensors by the window manufacturer, because the window seller threatened them with the loss of the window warranty when installing the sensors outside the factory (!). It is known that the more the seller sells, the higher the commission he has, so they try to squeeze as many "factory" options as possible to their customers.
Many of the mounted sensors did not work because they were mounted incorrectly. Window suppliers were usually from distant cities (because it was cheaper ...), so there was no way to count on their service (apart from the professionalism of the service itself and their knowledge of sensors ...).
To sum up - window producers have no knowledge about the types of sensors, methods of operation, principles of assembly and regulation. Just wanting to earn extra, traders press customers to install sensors at the factory, and then Mr. Kazio in production has a dilemma how to install it ...
Ultimately, the customer and the installer of the alarm system are left with the problem.
By the way, in large cities it is easier to find a professional installer of alarm systems and commission him to install sensors. There is usually a problem with this in smaller towns :-(
Regarding the cost of assembly, for me, the reed switch sensor costs 45 PLN net with professional installation and connection to the alarm system in a double resistor (2EOL) configuration, so if the manufacturer has similar costs to drill a hole and press any, the cheapest sensor, it is expensive.
This is what recessed reed sensors installed by us look like:

This is what the recessed sensors mounted by us in roof windows look like:

Here are two reed switches in the sunroof - they detect both opening and tilting the window:

This is what inertial detectors look like:

I inertial detectors painted to match the color of windows and doors:

One more thing, thieves rarely break the glass in the window, and if they do, it is in order to turn the handle and open the window, not to enter through the hole in the glass.
As a rule, a crowbar moves and the window is balanced - here, an inertial detector is activated immediately or a hole is drilled in the frame and a handle or a sliding hardware is opened.
When drilling in the window, the inertial detector should also work.
Regards
ZBYCH