Just saying and FWIW are similar that both make the statement a bit softer, however, FWIW sound a bit more neutral and polite. A simple saying can be more of a casual or even sarcastic statement, depending on the tone. To use an example, FWIW, I believe you will like this book is not offensive whereas Just saying, you might fail this test may sound offensive. FWIW means to provide information or advice to be considered, whereas just saying may be a casual comment which is at times teasing. Know more about fwiw meaning (https://radarro.com/fwiw-meaning/)