Our Sunni brothers they define Tawheed wrong way or they have understood incorrect meaning of Tawheed ( Oneness of Allah)
Any individual if you bring it parallel to Allah then it’s Shirk but if anyone calls any individual after Allah it means he has already accepted that Allah is the Supreme and when he believes that Allah is Supreme then now if anyone has any power or authority he will be considered as a creation of God and effect not cause. So if Allah is the final cause for everything then there is no issue if you believe anyone is the most powerful at last he will be the effect of the final cause (I.e. Allah).
Therefore if we shia say Ya Ali madad or Ya Abbas Madad it doesn’t mean that we say that God forbid they are parallel to Allah. No never. They will always be the creation of God and creation is always lower than creator and there is even no compatibility between Creation and Creator so one can compare them.
We do comparison between two similar things not between two things which has no match. Have you ever heard anyone says my garden is as much as beautiful as my Daughter???
So we can’t compare any entity with Allah even Prophets too because Prophets are also Allah’s creation.
But as Quran say
يَا أَيُّهَا الَّذِينَ آمَنُوا اتَّقُوا اللَّهَ وَابْتَغُوا إِلَيْهِ الْوَسِيلَةَ وَجَاهِدُوا فِي سَبِيلِهِ لَعَلَّكُمْ تُفْلِحُونَ
O you who believe! be careful of (your duty to) Allah and seek means of nearness to Him and strive hard in His way that you may be successful.(5:35)
Allah himself asks us to seek means of nearness because they are Allah’s closets Servants. And who can deny this that Prophets are not closets servants of Allah but the problem with them is they treat Imam Ali (a.s) like other companions of Holy Prophet and Caliph so they get confused after having listen Ya Ali or Ya Hasan or Ya Mahdi (a.s).