Pros and cons of planar transformers

[romovs]

I was looking for some low profile transformers and stumbled upon planars.
Surprisingly there is next to none information about them on the internet (or perhaps it's just me failing at googling).
Judging by the datasheets they seem to be: lower profile than toroids, higher price, higher efficiency at higher frequencies.
What about disadvantages? Are there any situations when planars should be avoided?

[SeanB]

Cost for one, they are only going to be used when you have no other choice as you need the reduced height and the current is high. Otherwise a plain ferrite core will be better. Often I have seen a standard low profile transformer mounted in a routed hole in the board and connected via the mounting pins to the pads or via short ends of the windings to get the extra 5mm of height reduction so as not to go planar. The planar are a lot wider as well, so take up more board space.

[WarSim]

Planar transformers all started with the notion of cost and space savings. 
Simple transformers turned out to be too inefficient.
Then more magnetic principles were introduced but these dramatically increased production costs.
There has be several such evolutions and papers on the subject. 

Over the years new understandings and realizations of limitations have been discovered. 
The original intent was to give the option for engineers to intrinsically include planar transformers in their projects.  Unfortunately the physics involved in creating efficient planar transformers proved to be beyond most electronics engineers.

Next was the idea to sell pre-designed planar transformers that still offered some of the space saving advantages without requiring magnetic design theory. 

What you can buy today is what the manufactures can produce for what they believe engineers will pay for the space saving benefits. 
So what you can buy is a compromise of the intent of planar,  manufacturing process, and efficiency. 
There have been allot more efficient and allot more expensive to create planar transformers made. 
This is not the end of the planar transformer it is still maturing. 

If you decided to used a planar transformer please read the datasheet carefully.  The selected manufacturing process may have surprising limitations normally assume with conventional transformers.