Here's my ambitious undertaking... in 2019, I'd like to take the bus from either Zagreb or Split to Sarajevo, before ultimately continuing on to Novi Sad. Does anyone have any experiences with the bus lines? I'm up for an adventure....but not too much of an adventure. I've seen the costs and timetables. I'd like to know what it's really like though.
In 2015 I bounced around the Balkans on buses and trains, but I barely touched Bosnia-Hercegovina, and I crossed Serbia (twice) east-to-west rather than north-to south. The only part of your route I covered was the short leg between Beograd and Novi Sad. Still, buses are pretty much buses, so perhaps this will help a bit.
My Croatian buses were modern. Elsewhere, they varied. Major international routes tended to have quite modern buses with comfortable seats and air conditioning. Some of the others were considerably older, with short legs between small towns sometimes using equipment not terribly different from school buses. You might also encounter short trips involving vehicles that are more van than bus. I don't specifically remember the Beograd-Novi Sad bus, but that's the kind of route that sometimes has older equipment. This year I traveled the short international leg between Szegad, Hungary, and Subotica, Serbia, and that was an older bus. I'm nearly certain that was a Serbian bus; the Hungarian buses I used elsewhere were much more modern.
I think you'll find consistency among the buses run by each of the large companies, and I wouldn't be surprised if all of your routes are on extremely modern equipment. You may be able to find specific reviews that mention company names on TripAdvisor or the Lonely Planet ThornTree Forum.
There will be a comfort stop every 2 to 2-1/2 hours. The driver will make an announcement about how long the break will be, but it will be in the local language. If you're lucky, your seatmate will speak enough English to translate for you. If not, you can catch up with the bus driver after you get off the bus and use hand signals or write "15?" or "20?" on a piece of paper and see what happens. Those stops are normally made at spots where you can buy food and beverages, not necessarily at bus stations. The locations seem to be selected because they have multiple, usually clean, toilets. Occasionally you may have pit toilets, but it's worth checking every stall since there may be a mixture. The stops made at actual bus stations may be quite brief (and I encountered some absolutely filthy toilets at a major station in Croatian Istria, of all places), so do not assume you'll have a long break there.
You may need small coins for the toilet-minder. You will certainly need local currency for food and drink. Both currencies may well work in border towns, at a non-optimal exchange rate.
My favorite bus-station snack--the name of which I unfortunately don't remember--was a savory cheese pie (like the Greek tiropita) that was cut from a rectangular sheet. The filling is a crumbly white cheese. It is oily and can be a bit messy to eat. Carrying a plastic bag can be useful here, because the cheese pastry will usually be handed to you wrapped in paper. Sometimes a sweet-cheese version is also available. Asking "sugar?" in a questioning tone in the local language will usually work; that would be, roughly, "secher?" The word for cheese, pronounced "seer", may also be useful.
You may be asked to pay a fee to store a suitcase in the hold of the bus. The charge seemed to be standardized and was the local equivalent of about $1.
I think I always had to get off the bus and walk into a border post for passport-checking. In my experience those are comparatively short stops with no opportunity to buy refreshments or use a toilet. There was just enough time for the bus passengers to line up and get their passports stamped.
If you don't speak a Slavic language, your ability to communicate may be more limited in Serbia (and maybe also in B-H) than in Croatia. I didn't run into the same large numbers of English-speaking young people in Serbia, though you're nearly guaranteed to have at least one on each bus. The linguistic skills of bus drivers and station employees are iffy at best, but it's really not difficult to cope as long as you don't have high expectations.
@ Will..."ultimately continuing on to Novi Sad." Fantastic! That is definitely accessible by bus from Belgrade.
Last year's trip on landing day in Frankfurt in the morning after the ca 11 hr non-stop flight from SFO, I took the train to Frankfurt Hbf to get a hot lunch, relax a bit at the station, before taking the train to Düsseldorf that afternoon.
As I was just about to leave my table (empty), 4 Serbs came by, one of the two younger women (in their earlier 40s) asking in German if the other 4 seats were taken or not. None of them could speak English, only that one woman spoke German, fluently, certainly not broken, obviously the translator for the other three. She told me not to rush, take it easy, etc (Bleiben Sie ruhig sitzen!). So, I decided to stay a bit, since I was in no hurry to get to the already reserved hotel in Düsseldorf. Hbf.
Anyway, I ended up asking them about Novi Sad since the German-speaking woman had told me all of them were from Belgrade, ie how to get there, what can be seen at Novi Sad, etc.
The old guy, ca late 70s, knew his history since he mentioned "Karlowitz" when he was talking in Serbian (that word I made out ) about Novi Sad to the three women, ie his wife plus the two younger ones. After he finished, the translator woman would tell me in German what was just said in Serbian..
Karlowitz was the treaty imposed on the Turks after being defeated by the Austrian Habsburgs under Prince Eugene in his campaign to drive out the Turks. In both Budapest and Vienna is a statue to Prinz Eugen.
All in all, we talked ca one hour, ...delightful, linguistically interesting, very pleasant.
We traveled on Flixbus in Croatia.
In Sarajevo and Beograd, we hired guides. In Beograd, we went to the TI and asked for a driver. We were referred to Djanni ("Johnnie"), who took us from Beograd to Novi Sad and from there to some of the small villages which were part of the Donauschwaben settlements that my grandma came from. It was very helpful to have him, since the small villages have people who speak Serbian, but not German or English. I work with German or English. He was reasonable in price.
Djannie drove us from Beograd to Sarajevo at a cost competitive (for 2) to the cost of airfare.
In Sarajevo, we again had a private driver take us to Mostar on a tour. In Mostar, we found that the buses had ended for the year (Nov 1), so he agreed to drive us to Dubrovnik. The situation was complicated due to visa issues as there was another couple which could not enter Schengen (they were from an Arabic country). So we drove thru the back country. Very interesting.
I can only report on the Croatian buses on the coast. The equipment was excellent, but the border stops often were lengthy because of a passenger or two in question. So they often were not on time.
We did this in reverse, starting from near the Serbian/Romanian border, taking the bus to Belgrade, then Sarajevo, then Mostar. We let ourselves be talked into a relaxing taxi ride for a really cheap price from Mostar to Cavtat (our base near Dubrovnik) but then we took the bus again to Split and then to Zadar where we flew out. This was all in 2012, we were in our early 30s at the time, not amazingly experienced travelers, first time in the Balkans, and we didn't find the bus travel a problem at all. It was a surprisingly comfortable way to travel, all told.