What are the effects If rituals after death are not performed at all or incorrectly performed?
What effects to the person who has died?
What effects to them who does not perform the rituals?
If Shraddhas are not done, then the deceased persons receive Pretatva or Preta-hood. That is they attain ghost like state of beings. Then, they will not be able to move to the Pitruloka and will be stuck somewhere in between.
I am quoting from the book Essence of Dhrama Sindhu (Dharma Sindhu is a synopsis of Hindu Dhrama and it quotes from Puranas, Smritis, Itihasas etc):
At the Bhojana Kaala, a Pinda be provided to the Preta by addressing it by the Naama Gotraas. It is also considered essential to place at the ‘Mrita Sthaana’ some cash to defray the travel expenses as the Pretaatma travels from Bhu loka to Para Loka!) Antya Dainandina Shraaddha Vidhi : The Karta is required to execute ‘Nava Shraddha as’ on the first, third, fifth, seventh, ninth and eleventh days.These Nava Shraddhas are prescribed by Rigvedis as also Apastamba Shutraites. It is stated that within the ten day period Nava Shraddha s be performed and Nava mishra shraddha be done in the year long period.
Akrutwaatu Navashraaddham Pretatvaa- nnaiva muchyatey,
Nava Shraaddhgam Tri Pakshamcha shaanmaasika Maasikaanicha,
Nakaroti Suto yastu tasyaadhaha Pitarogataah
Pretatwaa would not be liberated if Nava Shraddhas are not performed by Putras and what would be worse if besides Nava Shraaddha, the Tripakshika and Shaanmaasika are not done, then the Pitraadis would definitely go down to ‘Adhogati’or the lower abodes.
And, the sons who have failed doing the Shraddhas are also cursed and targeted by the Pitrus as stated below:
Even if there is some deficiency of Sankalpa Vidhi and Pinda Da anas, Bhojanaadis are indeed the essentials. Brahm anaarchana is very important to Rigvedis, Pinda Da ana is essential to Yajurvedis and Saama Vedis. In any case :
Ashraddhayaa Pitaro nasanteeti matwaa Shraddha kurvaanasya riktam Pitarah pibanti - Those who have Naastika Buddhi and Ashraddha or dis-belief and intolerance ignoring the Shraaddha Karma would be targetted by Pitru Devatas by drinking their blood eventually!
Anyways, not performing Shraddhas is considered as very bad. One who is not doing so, as prescribed in scriptures, is regarded as someone who is permanently impure. That is, they are not considered fit to perform any other sacred rites.
Daily impurity is for one who is addicted to gambling, etc., and for a dependant. The impurity of a person, who does not perform the S'raddhas, ends with his ashes (i.e., death). (10)
Temporary impurity is not for them, but a lifelong one. Thus impurity according to the differentiation of merits has been spoken of. (11)
Daksha Smriti 6.10,11