Those who worship demigods enter into the darkest region of ignorance.
Śrī Īśopaniṣad Mantra 12
Those who are engaged in the worship of demigods enter into the
darkest region of ignorance, and still more so do the worshipers of
the impersonal Absolute.
Vishnu worship is the religion of Gods while Shiva Worship is the religion of Daityas and Rakshasas.
Vaman Purana 11.15-16,26
Gods other than Vishnu should not be worshipped
You alone are the lord of brāhmaṇas. No other god is respectable. O
Puruṣottama, those who worship any other god than you, have become
impious, and are censured by all the world. You, Janārdana, are alone
fit to be worshipped by brāhmaṇas, well-versed in the Vedas. None else
among the gods is ever venerable.
Brahmā, Rudra and others, mixed with rajas and tamas are not fit to be
worshipped. You, having pure sattva, are fit to be worshipped by
brāhmaṇas. The water (flowing) from your feet should be resorted to by
manes, gods, and all brāhmaṇas. It gives salvation, and destroys sins.
Manes, gods, brāhmaṇas should take what is left over after you have
eaten, and not what is left over after others have eaten. The food,
flowers, so also water of (i.e. offered to) other gods, would all be
unfit to be touched. It is useless like liquor. Therefore, a wise
brāhmaṇa should constantly worship you, the eternal one, everyday and
should take water of (i.e. sanctified by) you and food eaten by you.
65-68. A brāhmaṇa should not see another god; should not worship him.
He should not partake of another god’s favour; should not enter the
sanctuary of any other (god). (If) a man does not give in this world
the food and holy water enjoyed by you in the rite of śrāddha offered
to his ancestors, all that would be fruitless. His dead ancestors fall
into (and live in) the hell of pus and blood for thousands of crores
of kalpas and hundreds of crores of kalpas.
Padma Purana Uttarakhanda Ch 255
Shiva is the God of Tamas Guna. Since Shiva is composed of the Hymns of Sama Veda, The sounds of Sama Veda are impure.
Markandeya Purana 99.19
The Vicious ones worship Shiva as the highest God Which leads to their downfall.
The lord said:
O Rudra, O you of mighty arms, O you best god, for deluding the
enemies of gods prepare a course of conduct to be followed by
heretics. Narrate to them the dark Purāṇas (i.e. Purāṇas that will
take them astray). O you very intelligent one, also fashion confusing
holy texts. There will be brāhmaṇas and great sages who have given
themselves away to me.
26-33. Approaching them through devotion for me, tell the vicious
ones: Kaṇāda, Gautama, Śakti, Upamanyu, Jaimini, Kapila, Durvāsas,
Mṛkaṇḍu, Bṛhaspati, Bhārgava and Jāmadagnya are the ten (?) vicious
sages. Desiring to do the good of the world, approach them with your
power of suggestion (bhāvaśakti?). By your power increased in
viciousness infused into them they will be vicious in a moment. There
is no doubt about it. Those brāhmaṇas with greater viciousness
caused by you, will narrate the vicious Purāṇas and doctrines in the
three worlds. O god, you yourself, wearing the marks like a skull,
hide, ash, bones, fully delude the people in the three worlds. So also
you, Śiva, prepare the Pāśupata order with its divisions like Kaṅkāla,
Śaiva, Pāṣaṇḍa and Mahāśaiva; so also properly prepare the doctrine
having no particular marks and outside the Vedic fold. All the mean
men will wear ash, bones etc. and will be insensible. The vicious ones
will describe you as the highest (god).
34-36a. Taking to their doctrine all the ancient demons will be
averse to me in a moment. There is no doubt (about it).O very powerful
Rudra, in my incarnations I too shall worship you in every age to
delude the vicious ones. Having upheld this doctrine, they will
Padmapurana Uttara khanda Chapter 235
Worshippers of Shiva will descend to the standard of atheism and be considered heretics due to the curse of Bhrigu muni.
By blaspheming the principles of the Vedas, which are the pure and
supreme path of the saintly persons, certainly you followers of
Bhūtapati, Lord Śiva, will descend to the standard of atheism without
Srimad Bhagavatam 4.2.32
Thus repudiated by him the (sage) of a great penance remained there,
at the door of the great lord, for many days. “Since he, arrogant due
to the company of a woman, thinks lightly of me, he shall have the
form of clitoris. Approached (i.e. overcome) by vice he slights me, a
brāhmaṇa! Reduced to an impious state, he will not be adored by
brāhmaṇas. Therefore, all the food, water, flowers and oblations
offered to him will undoubtedly be useless.”
35-40. The very lustrous one, having thus cursed Śaṃkara honoured by the world, said to the very fierce attendant, Nan-din, holding a
spear, O king: “May Śiva’s devotees, having ash, phallus and bones be
impious and out of the Vedic fold.”
Padma Purana Uttara Khanda Ch 255
Shaiva puranas lead to hell.
18-22a. Know from me that Mātsya, Kaurma, Laiṅga, Saiva, so also Skānda and Āgneya are tāmasa (vicious). O you of an auspicious
appearance, the Purāṇas, viz. Vaiṣṇava, Nāradīya, so also the
auspicious Bhagavata, similarly Gāruḍa, Pādma, Vārāha should be known
to be sāttvika (or virtuous). Know from me that Brahmāṇḍa,
Brahmavaivarta, Mārkaṇḍeya, Bhaviṣya, Vāmana and Brāhma are rājasa
(endowed with the quality of passion). The sattvika ones are said to
give salvation and are always auspicious. Similarly, O goddess, the
tāmasa are said to be the cause of (i.e. lead one to) hell.
Padma purana Uttarakhanda Ch 236
Those Brahmanas who eat food offered to Lord Shiva will be roasted in hellfire.
98-101. Therefore, brāhmaṇas should always eat the remnants of the
food offered to Viṣṇu. The remnants (of food offered to other gods)
are useless and (therefore) condemned. A brāhmaṇa, weak in knowledge,
who even once eats the remnants of food etc. offered to Śaṅkara etc.,
would certainly be a cāṇḍāla. For thousands of crores of kalpas he is
roasted in the fire in hell. O best brāhmaṇas, the remnants of
offerings to gods like Rudra, are said to be equal to food offered to
demons, yakṣas, goblins and like liquor and flesh. Brāhmaṇas should
not eat that food, the offering enjoyed by (these) gods.
Padma Purana Uttarakhanda Chapter 255
Even Shakta Upapurana Devi Purana says Shiva's prasadam is not fit for consumption,only Vishnu's Is.One who consumes Shiva's prasad Faces Vishnu's wrath. Shiva's prasad becomes fit for consumption only when purified by touching it with a Shalgram Shila of Vishnu.
Devi Purana Chapter 81
Those Who Worship Brahma, Shiva etc. undergo great hardhips, calamities and foul diseases like Leprosy or Fistula. Hence, not even in adverse circumstances, Shiva should be worshipped.
Garuda Purana III.1.27-28
Those who condemn the worshippers of False Gods (i.e Gods Other than Vishnu) are truly Good people.
These mantras are thus explained in the Kūrma Purāṇa:—“Undoubtedly the
worshippers of other deities than Viṣṇu go to blinding darkness, but
undoubtedly to greater darkness they go who do not censure and condemn
such persons (and fail to??????????? their mistakes). Therefore those,
who know the Lord Narayana, in His true form as free from all evils
and who also condemn the worshippers of false deities, are truly the
Kurma Purana, Quoted in Madhvacharya's Commentary on Isopanishad Mantra 11