Let me fill you in:
Double glazing is made up of two layers of glass, with an air gap between the layers. It is this gap that provides the insulation properties of double glazing. The glass can be any combination of clear or tinted, annealed (normal), laminated or toughened glass. Sometimes the air inside is replaced with Argon gas to enhance the insulating performance. One layer of the glass can also have a Low-E coating to either keep more heat in, or to keep it out. All around the perimeter, between the glass panes, is a "tape", usually with little holes. Behind the holes is a desiccant to absorb moisture.
If a film is applied to the room-side of the double glazing, this pane will warm up in the sun. How warm it gets, depends on the film type used. Non-reflective film, such as car tints, have a high absorption rate of up to 70%. As the glass heats up, it expands. It also pressurises the air between the two panes. The risk here is two-fold. Firstly, certain glass types can crack. (Thermal stress) Secondly, there is a risk of air getting into the gap. This may only happen very slowly, but over time, it will show up as condensation between the glass, as the desiccant can only absorb so much moisture. This can happen without film being applied, but "stressing the glass" will accelerate this process. (Seal failure)
We have to use a film that has a low heat absorption to minimise the risk of thermal stress and seal failure. Generally, SV25, SV50 or the new Daydream range are the best film types for this.
So, if we suggest certain window films, it is not because we like them, but because they tend to be the safest options.
Please feel free to ask questions about this, or any other aspect of window tinting.