I'll mention a criticism which derives from Leibniz. There is not total disagreement between Spinoza and Leibniz on necessitarianism but their views irreducibly diverge as we cut deeper. In my view Leibniz's critique amounts to a major and sound criticism.
One of the supporting pillars of Spinozas system is the idea that the world does not
admit of changes: that natural reality is a package deal as is, and it will not admit of
being changed. And here Leibniz is prepared to meet Spinoza halfway. For Leibniz
concedes to Spinoza that all salient features of the real world are reciprocally co-necessary: given one such fact all the rest are correlatively constrained to fall into place.
For Leibniz, the substances of each possible world are reciprocally adjusted to one
another in a comprehensibly all-inclusive total way. To use one of Leibniz' favorite
metaphors, the substances of a possible worlds "mirror" one another in and all-inclusive mutual accommodation. Within a world no change is possible - given one
of its constituents everything else is correlatively necessitated. Internally, within any
individual Leibnizian possible world, all arrangements are every bit as necessitarian as
Spinoza has it with regard to the existing world. But that does not render that world
as externally absolutely necessary so that the entire totality of fact has no alternatives.
And so for Leibniz substances - be they actual or possible, real or phenomenal - are
unchangeably what they are. But their existence - their presence in the actual word of
existing reality - is another matter altogether. It is with existence rather than description that contingency comes upon the scene. (Nicholas Rescher, 'Leibniz as a Critic of Spinoza', Studia Leibnitiana, Bd. 47, H. 2 (2015), pp. 186-204: 189-90: 189.)
At this point we come to a very fundamental parting of ways between Leibniz and
Spinoza. For there is a crucial difference between excluding change IN the world and
excluding change OF the world - that is, between alteration and replacement. For so
Leibniz maintains, there could possibly exist a different world altogether, one whose
realization would involve not modification in but outright substitution for the prevailing order of things. To illustrate: Say that an impresario is going to give a reading of
William Cullen Bryant s Thanatopsis. Obviously he cannot make changes within the
poem - when altered it is no longer Bryant Thanatopsis. One could abandon Thanatopsis altogether and present on an alternative poem. The fact of "internal" unalterability does not preclude the prospect of "external" replacement. And just this is Leibniz's
idea: the immodifiability of the existing world does not preclude the prospect of its
total replacement by another: it is only one among many alternatives.
For Leibniz, these logico-metaphysical considerations mean that alternative frameworks of existence indeed are possible. Intra-world unchangeability does not preclude
transcendental (world-exceeding) alternativeness. Reality is one alternative among
many. Even though nothing can be changed within the universe, nevertheless the universe-as-a- whole could in theory be exchanged for an altogether different manifold of
existence. Denying changeability within the world does not thereby deny the change-
ability of the world as a whole. To reject changeability is not to reject exchangeability.
Tracking the implications - why is the world as it is?
The different approaches to alternative possibility have profound implications in the
point of corresponding differences in metaphysics. For Spinozas approach enables
him to avert addressing the question which Leibniz takes to lie at the heart and core
of metaphysics: Why is the world-as-as-whole as it actually is rather than somehow
For Spinoza the real world as we have it necessarily as is: no alternatives to it can
rationally be envisioned and there is nothing else - nothing world external - to provide an explanatory account for its existence and its nature: like the God of traditional
Christian theology it is causa sui carrying within itself the explanatory basis of its detail. And so, for Spinoza the problem of the nature of existence is itself nonexistent:
reality is at is because it necessarily has to be so, there just is no alternative for it.
Instead, for Leibniz, the character and existence of natural reality cry out for explanation because they are not necessitated but contingent. Seeing that there are alternatives why should it be that this particular manifold of possibility is the one that exists?
Accordingly, Leibniz holds that it lies in the very nature of this question that the alternatives explanation of the real world must itself be extra-mundane. For if circularity
is to be avoided - if the explanation in question is to avoid a rationally vitiating circularity - then the world s explanation must lie in something altogether independent
of and extraneous to the actual world itself. Seeing that the word is contingent its
nature requires an extra mundane explanation. And so, as Leibniz sees it, in failing
to acknowledge the world s contingency Spinoza fails to confront philosophy's most
central problem: why it should be that the realm of Reality is what it is. Rescher: 189-90.)