I heard that Imam Qasim bin Ibrahim (radia'Allahu 'anhu) criticised Imam Ali al-Hadi (who the Ithna Ashariyah claim to follow). Could anyone provide me with more information regarding this insha'Allah.
And is there any other criticism towards any of the later Ithna Ashari Imams?
May Allah guide us all unto the Sirat all-Mustaqim.
Yeah. From what I can remember, in the book Radd ala Raafida, Imam al-Qaasim (as) criticizes one of the Rafidite imams for misappropriating the money of his followers. I dont think he mentioned Imam Ali al-Hadi's name per se. However, it is assumed that he was talking about Ali al-Hadi because he was the imam of the 12ers at the time. The only person whose name he mentioned was Hisham bin al-Hakam, the chief theologian of the 12ers. He charges him with tajseem and other things.
I dont think that the Zaydi imams (as) attacked the 12er imams per se. Rather, they attacked misconceptions about them.
The most in-depth refutation of the 12er doctrine was Al-'Iqdu ath-Thameen fee Tabyeen Ahkaam al-A'imma al-Haadyeen by Imam Mansur Billah Abdullah bin Hamza, alayhi as salaam.
Allah knows best!