Grimm's Law is about to expire (Collinge 1985, p. 267, Thundy 1991)

"On the contrary, I (Z. Thundy) hold that all scholarship, especially
study of the origin of the language families, is very tentative. This
qualification applies to the laws of Indo-European, particularly
Grimm's Law, which governs the reconstruction of the consonants of
many proto-Indo-European roots.

Most Indo-Europeanist cite the many laws of Indo-European as gospel
truths even though scholars have fought and continue to fight over
them, and there remain many honest doubts about them. Some of these
laws, as Collinge puts it, "have sickened for years and many be beyond
salvation: those of Fortunatov (I), Lachmann and Thurneysen are cases
in point" (The Laws of Indo-European, Philadelphia: Benjamin, 1985, 2).

The reason I (Thundy) am skeptical of many proto-Indo-European roots
based on Grimm's law is that most Indo-Europeanist apply Grimm's law
universally and claim that there is no exception to it. For the Old
English habban, dictionaries give *kap as the root and ignore the
Latin habere as cognate, because Grimm's Law does not allow for any
exception! Then we have the classic cases of Sanskrit pibati, of the
Greek pachus versus the Sanskrit bahu, and the Latin bibit versus the
Praenstine pipafo (Collinge 259-65). By postulating laryngeals we can
get around these problems, as scholars have done. IT IS LIKE
RECONSTRUCTING LANGAUGES FOR THE SAKE OF LAWS RATHER THAN USING LAWS
TO DESCRIBE LANAUGES.

Of course, lexicographers are not going to change all their
reconstructions nor are traditionalist willing to give up Grimm's law
without a struggle, even though Grimm's law is about to expire
(Collinge 267) with the radical revision of the proto-Indo-European
obstruent system by the glottalicists Gamkrelidze, Ivanov, Hopper and
Bromhard.

My (Thundy's) main reservation about lexicographers' reconstruction
method is that it is too rigid. The reconstructed roots indicate that
the reconstructionists are working under the assumption that
proto-Indo-European, even when it was spoken (?), was a simple,
self-contained pidgin language with no dialects and no interaction
with other language families. All of the language families we know
are constantly interacting with other language families by giving and
receiving among thesemselves. Unfortunately, we have no literary
evidence for proto-Indo-European and its relation with other language
families. Since every language comprises dialects, consistent voicing
of consonants is not necessarily an infallible criterion for
distinguishing dialects or language families—in American English, for
instance, the voicing of /s/ in words like greasy is at best a
variable dialectical or ideolectal feature. That is why I am not so
keen on accepting jush instant of cush, for example as a cognate of
gustare. Maybe many of the supposed cognates are rather synonyms; we
shall never know (Thundy 1991, p. 310, emphasis added)."

Knowlton, E. C., Thundy, Z., and Galloway, A. (1991). Philology and
Anglo Saxon Poetry. PMLA, Vol 106, No. 2 (March), 308-312.