If you go through the books of ahnaaf, you will find different answers when one misses a strongly emphasized sunnah prayer. It seems there is also a difference in the hukm between other acts that are deemed sunna mu'akkada, and the sunnah mu'akkadah prayers.
You have texts stating that one is only exempted from the intercession of the Prophet. So is this a sin or not if you are only left out from intercession?
If yes, the hadith mentioned that intercession is for the sinful people?
Then you have texts stating that it should not be left continuously without excuse. One sins only if it is done continous. But what is meant by that? If you miss it for a whole year? Or 1 time a day? Or once a week? A month?
Also, it is unclear what an excuse really means? If you are at work? Or busy with worldly things? Or do we mean with excuses, like the excuses mentioned by the fuqaha in order to miss the jama'a?
Then you have texts mentioning that it is a sin even if you only miss it once. They state it is obligatory to pray such a salah. A recent fatwa on : "It is compulsory to perform a sunnah mu’akkadah salāh." See here
I came across also this fatwa by Shah Abdul Aziz in Fatawa Azizi:
The sunan rawatib that are performed in the 5 prayers, the masses think that it is so clear (mustahkam) that all sunnah prayers are entered in the fard. While except the sunnah prayer of fajr, the remaining sunnah prayers are not emphasized in the hadith, and many muslims, men and women considered it difficult to pray because of the many rak'aat. So if only those 17 rak'aat in day and night that are fard are ordered to perform, then the people will easily perform it.
The scholars of what is beyond the river (ma wara an-nahr) were extremely harsh about the sunna prayer, so much that the ignorant amongst the masses considered the sunnah prayer close to obligatory (fard). So much emphasis is not proven from the hadith and this is the research of our father. This is also established from the ahadith and athar. The scholars of behind the river were harsh (tashaddud) in emphasizing the sunnah prayer, that the awwaam considered the sunnah prayers as fard. Our father would say that this is a kind of tahrif of the islamic law i.e. considering the sunnah as fard is a tahrif in shariah. (Fatawa Azizi p. 493)
Online Urdu translation of Fatawa Azizi