For the infobox Category, use the lowest level category available. In other words, use Limb-mounted.
Some of the information (battery life, connectivity, description of operation) in the introduction summary (the very 1st paragraph) is better suited for the Main characteristics section.
Some of the values in the infobox are incorrectly formatted. The Sensors category should have a semantic property for each of the sensor individually. The same goes for the Operatin system category. You can find how to fix it in Semantic_wiki_formatting#Dealing_with_multiple_values.
Don't write in second person narrative ("an application on your smartphone"). Use informative and neutral/passive writing style.
I'm not sure about using Czech language source. We'll have to discuss this with Ms Žáčková next time.
Great job with the Main characteristics section.
The Important dates section is empty. Don't forget to add relevant references when you will be writing important dates.
Proofread your text. There are some strange expressions such as "irritation has been appeared at users wrist".
The data in the infobox needs to have references added.
Important Dates is missing references for the individual dates.
The Health Risks section is perhaps a bit convoluted. I know what you are trying to say, but rephrase your sentences so the text is easier to read. No need to use complicated sentences at any cost.
If there is a source that states Fitbit is not recognized as a medical aid, please reference it. If not, it's okay to leave it without a reference.
The fact that Fitbit "is a term" is irrelevant. Focus on the presentation itself. If there are articles that call Fitbit to be something, write about it, and reference it properly.
Do not use expressions like "it seems". Either you are certain or not.
Public Policy section. Do not use expressions as "should be". Use informative tone. If there is no legislation about user privacy, simply state it in a neutral voice.
It's okay if there are no related technologies or research but state the fact too. A section saying that there is no information is still better than an empty section.--Haustein (talk) 16:20, 11 December 2015 (CET)