Perhaps you are right.
But I see that there is a huge gap between what the theorists of linguistics say how comparative linguistics should be done and how it's actually done. There are quite a few etymological dictionaries that, for instance, connect "hormone" with "are", even though there is no conceivable IE root that would give, by regular sound changes, both words (s-mobile can't be in front of a laryngeal). And I've seen many examples of IE roots being invented just to explain two words in different IE languages, often including one toponym. Many theorists of linguistic science say that a paper that argues that something is an IE language by showing close phonetic proximity between its words and words in various IE languages would never pass a peer-review, and I see it does. Many people say that relatedness of languages are shown by regular sound correspondences, when, in fact, Hittite has been shown to be IE by its word for water. Comparative linguistics is done by trial and error of people relatively uneducated in it, regardless of what the theorists say.
Anyway, I am trying to help the Croatian historians by interpreting the toponyms. Many toponyms appear to be easily explainable by PIE. Which is to be expected, IE languages have been spoken here ever since mid 3rd millennium BCE (Vucedol culture). There a few astounding examples. The ancient name for the river Kupa is Colapis, and that's obviously *kwol+*h2ep (water with meanders). The ancient name for Zagreb is Andautonia, and that can quite easily be h2en+dheh2+u(n)t+on(=om), so that it means "by that which flows". However, the mainstream Croatian toponymy quite often doesn't appear to have looked into PIE. Issa, the ancient name for the island Vis, is widely stated to have an unknown, perhaps Pre-Indo-European, etymology. However, it can easily be derived from *yes+*eh2, in the sense "where a lot of springs are". There were spas there in the Roman times. And it appears that all the ancient names for the places in Croatia where the Roman spas were share the same root. Daruvar was called Balissa (I believe Bal means bright, from *bhel) and Varazdin was called Iasa. There are multiple rivers and streams whose names appear to be derived from *h3rews. On Risnjak, the mountain, there is a stream with the same name. Many people say that the stream was named after the mountain, although it could easily be the other way around. Also, the ancient name for the river Rasa is Arsia, and, in Slavonia, there is a stream called Ervenica. Cibalae, the ancient name for Vinkovci, could easily be from *kjey+*bel (strong house), and it seems to me that nobody suggested it. The IE word for valley, *h1eyn, also appears in multiple toponyms. Incerum, the ancient name for Pozega, is often said to have an unknown etymology. However, it can easily be *h1eyn+*kjer, so that it means "the heart of the valley". The ancient name for Donji Miholjac is Mariniana. It could be from Marinus, a common roman name, but it's more likely a Latin folk-etymology of *mreys+*h1eyn, "marshy valley", which is what Donji Miholjac actually is. The mountain Papuk is said to be named after the Papuk stream, but the stream is said to be of unknown etymology. I believe it is actually from *bhebhogj (repetitive participle of *bhogj, "that which flows and flows"). The mount Psunj is also said to be of unknown etymology, even though its ancient name, Pisunus, is very similar to the PIE word for resin, *pisnu, and Psunj has a lot of softwood. The river Sutla is also said to be of of unknown etymology, although it can very easily be *suh1nt, participle of *sewh1, so that it means "that which waters the ground". Pazin is also said to be of unknown etymology, even though it's sensical as *ph2senti (pasture) There are many toponyms which are more sensibly explainable using PIE than using SC. Mainstream etymology connects the river Vuka with the SC word "vuk", for "wolf". However, it's more likely from zero grade of *welk (a PIE onomatopoeia for "to flow", syllabic l often vocalizes to u in SC even in today's loanwords), isn't it? Baranja is usually derived from "baran", a spurious SC word for lamb, but isn't it more sensical to derive it from the PIE word for marshland, *beh3r? The river Orljava is said to be derived from SC word for echo, "oriti", but isn't it more sensical to derive it from *h1or (to flow)? There are some villages whose names mainstream etymology derives from "daleko" (far away), like Dalj and Daljok. Isn't it more sensibly derived from *dhel, in the sense "milkmen"? Though, there are some place names that would appear fanciful in PIE. The ancient name for Valpovo is Iovalum, which would mean "magical beer" (*yow+*h2elut). There was quite a demonstrable word there, something like *ker, meaning "to flow", occurring in many streams and rivers (Krapina, Karasica, Krka, Korana, Krndija [the stream]…), without an obvious IE root. Or the suffix *-la in the river names like Orljava and Sutla. There are some toponyms which multiple languages could give a sensical explanation for, for instance, Pannonia (both Latin "pannis" and PIE *pen appear as sensical origins). I've tried to reconstruct some grammar of the ancient language of Slavonia based on the toponyms. Obviously, it was a centum language. I believe it had an ablaut, but not with the vowels e and o, but with a and u. For example, Mursa (the ancient name for Osijek) and Marsonia (the ancient name for Slavonski Brod) obviously share the same root (probably *mreys), and Papuk would then be a grammatical repetitive of *bhogj. Because of the epenthetic vowels (Ervenica), I'd suggest that the accent was on the first syllable (as in Ancient Greek "aster", "oros" or "ennea"). Do you think I am doing it right?