Rashi and Tosafos disagree about the practice by the wars of Dovid, how the gett would be written. Rashi holds that it was written conditionally so that it will go into effect if the husband fails to return at the end of the war. Tosafos holds that it was written unconditionally, just that it was done very privately so that no one will try to marry her. The gemara refers to a woman who received such a gett as a "safeik eishes ish". According to rashi it is literal since it is unclear during the war whether the husband will return, but according to Tosafos she is definitely not an eishes ish, but she is called a "safeik eishes ish" since the gett was done quietly and no one is aware that she was divorced (Tosafos is difficult - since this was the common practice, surely people could assume that it was done. Why is it considered b'tzinah?). Maharam Shiff points out that Rabbeinu Yechiel Mi'Paris banned conditional gittin even for one who is dying, based on this Tosafos.
Tosafos concludes with a question on both himself and rashi. The gemara says that causing someone embarrassment is worse than a safeik eishes ish, which implies that having relations with one who is definitely an eishes ish is still worse. However, Dovid's logic as to why it is worse to embarrass someone - for relations with an eishes ish the punishment is strangulation but he still has a share in the world to come, but for embarrassing someone one loses his share in the world to come - is a valid rational as to why embarrassing someone is even worse than a definite eishes ish. So, why does the gemara only say it is worse than a safeik eishes ish?
The Rashash answers this question by referencing a Rabbeinu Yonah at the beginning of brachos (it is also found in sha'arei teshuva as well as his commentary on Avos in the first perek). Safeik in a certain sense is worse than vadai, because when one does a definite aveira they realize the severity and will be inspired to do teshuva. However, one who does a safeik aveira will forever justify their actions and never do teshuva. Based on the notion that a safeik is worse than a vadai, perhaps the gemara chose "safeik eishes ish" to accentuate the point that embarrassing someone is not only worse than vadai eishes ish, it is even worse than safeik eishes ish. The maharam shiff has a similar approach in dealing with this issue (just that he explains that a safeik is worse than a vadai since a vadai has a specific punishment through which one achieves atonement but safeik doesn't have that).