It seems to me that some words that have -p- in stem in Latin have clearly reconstructible -ku̯- based on other Indo-European languages. Some examples include
- *u̯lpos - *u̯lku̯os ("wolf")
- *u̯esper - *u̯esku̯er ("evening")
- *apa - *aku̯a ("water")
- *i̯epr - *i̯eku̯r ("liver")
The "ku̯" reconstruction is supported by the majority of languages, still many authors consider the roots with "p" as genuine and/or separate. For example, Katz tryed to proove the connection between *u̯esper, Hettite *u̯esp "cloth of the dead" and Greek ospros "pulse".
Mallory & Adams reconstructed the PIE water deity as *H2epom Nepots instead of evident *Aku̯am Nepot(s) (note that the laryngeal here is not supported by the Nostratic evidence).
Starostin lists both *ap- and *aku̯a; *welp- and *welk- (and also *lup-) as separate roots.
I wonder why the correspondence between **ku̯* and **p* is not evident to some authors?