The Good, the Bad and the Ugly
Ever wonder what's inside those "sealed" baluns? Certainly seems there are many manufacturers who would prefer you not know. Over the years we have collected inside pictures of these units and here are a some examples for your consideration.
First is our model 4115 which is a dual core, 5kW 4:1 current balun.
Here are some of the same 4:1 models from other companies.
Next we have our model 4113 which is a single core, 4:1 current balun, 3kW unit. 
And again those that would call theirs the same.
Then of course there is venerable 1:1 current/choke balun which is also a feedline isolation and RFI suppression balun Here are models 1115 and 1116d.
With competitive offerings for your comparison.
Lastly is our 4:1 Hybrid balun model 4116
And of course the competition's offering .... yes that is spray foam insulation that has hardened.
In the immortal words of John Ruskin, "There is hardly anything in the world that some man cannot make a little worse, sell a little cheaper and the people who consider price only are this man's lawful prey".