The Roads in Serbia, and especially Romania are ok! Not France-like, but good by Balkan standards.
You are in Croatia now? Well, there you have it. Serbia is similar (less motorways), Romania is more of a mixed bag, from brand new excellent to relatiovely poor and bumpy - but still paradise compared to ex-Soviet countries.
Who told you they were really bad?
As has been said before, Ukraine is more of an issue - to a lesser degree also Albania, especially off the beaten path.
In UA, huge potholes the size of Ladas even on the main arteries are the norm, cars heading almost smashing into your face at the last moment on the wrong side of the road to avoid said potholes.
Romania has more or less redone all major roads, to the point of sometimes being better than in some Western countries.
Yet, some passes where under construction last summer - sadly, these will now be EU-Autobahn-like, rather than the beautiful tight mountain twisties they used to be...
However, you'll immediately notice a vast step forward in road surface quality when coming in from ex-Soviet countries (Ukraine / Moldova). And no step into the 3rd World if you come in from ex-Yugo / Serbia / Croatia - just more variation compared to ex-Yugo.
Montenegro takes the cake, better riding roads than anywhere else in SE-Europe, including all EU countries in the region (Croatia included). I love this place!
To compare: Some Greek roads aren't necessarily any better than RO in the remoter areas, on the contrary - many more surprises possible in Greece (trees and bushes growing on the centre lane divider, land slides and rocks in curves and other such things...). Yet everything is ridable!
Yet, these new main roads in Romania and Serbia aren't that much fun to ride, in my opinion - way too much traffic, relatively frequent police checks (radar!), I managed to mostly avoid them by having a lot of time on my hands, and setting the Navigation to "curvy" or "direct route / small roads preferred".
More surprises possible on B-roads in RO than in Croatia (unless you get to the remote parts of the Knin area, where they tend to "forget" putting on tarmac in Serb minority areas, and you end up riding 50km on gravel...), but still no prob whatsoever.
I'll make an exception in the Tara valley (Montenegro) because of the incredible scenery, or along Danube Serbia-Romania border (go ride the Serb side at the Danube Iron Gate, no traffic at all, Romania side of the Gate is jam packed solid with cars, trucks and buses in summer!), but still - try to take your time and plan in smaller country roads.
You'll get to know the country much better choosing B-roads and zig-zagging between villages, than by following the Navigation System's fastest choice for big city A to big city B! Those big roads are no fun (for me).
Smaller side roads in RO and SRB are so much more rewarding than taking the busy and mostly boring main highways! A few more bumps and some animals are to be expected though - lack of traffic and much nicer scenery will more than make up for it! Also, if you are in a hurry, better be on side roads than on the main ones (lack of radar cops is always a bonus...)
Side roads aren't that bad anymore, just take care at any railroad crossings anywhere in RO - every car and truck will stop for good to a screeching halt before crossing them, even if they look ok to cross at 80km/h (it may be a law?). Even if there is no train to be seen anywhere, or if the rail line seemingly out of use - they will put the brake pedal to the floor....
Sometimes, these rail crossings can be very bumpy, so do as the locals do, stop to a halt!
Also, horse carts are a very frequent sight all over rural Romania, so take care when riding around blind curves in the countryside - but that is true anywhere in the world.
In Serbia and all other ex-Yugoslavian countries, its IMT tractors and sometimes loose cattle or a herd of sheep, rather than horse carriages.
And yes, both Romania and Serbia are best done on a regular street bike, even the Ukraine (albeit at extra care to watch out for huge Soviet potholes and crazy pothole-avoiding Lada & ZiL truck drivers).
I did several 8000km Cross-Carpation and Balkan tours on a Suzuki GSX 1200 naked bike. In 2013, 2015 on a KTM Duke 690 to Kosovo, spring 2016 to Greece, summer 2016 all along the Carpatians, as well as this spring 2017 on my Suzuki to Greece - basically up to 2months / year riding down there in the past few years.
Routes, with variations: Germany - CZ -> Austria -> Slovakia -> Poland -> Ukraine -> Romania -> Serbia -> Montenegro -> Kosovo -> Macedonia -> Greece -> Albania -> Montenegro -> Bosnia-Herzegowina -> HR -> Slovenia and back home across the Alps.
Including some voluntary offroad sections - I sometimes prefer to be off the beaten path, despite lack of an enduro.
Nothing to worry about, I had a lot of fun riding fast up and down the twistie mountain passes in Southern Serbia on my KTM Duke 690 in 2015!
In Romania, take your time to explore the remote areas - off the two main Carpatian mountain passes Trasfagaran and Transalpina.
In summer vacation season, I considered these two Trans-passes as overrated obnoxious tourist traps. Was a little disappointed (well, I live in a hilly area near the Vosges mountains and the Alps, maybe that's why?)...
Prepare for endless traffic jams of buses and tourists in cars and ultra-slow groups of Goldwings / GS1200's on their "trip of a lifetime".
I got the f*ck out of there, way too crowded for my liking - I had much more fun times roaming around off the beaten path around the mountains in North-Eastern RO (Moldovia region inside RO).
Moreover, the weather situation can be very unpredictable in the Arges mountain region (where these passes lie), expect a higher chance of sudden heavy rainstorms and freak fog in high summer than almost anywhere else in this part of the world.
My advice: Get your expectations down on these two tourist trap passes, and do make time to stop at other places as well, for example the Northern Romanian Carpatian range, or the Hungarian speaking mountain area in the centre of the RO Carpatians.
Or stroll to a stopover at a small city like Alba Iulia down in the planes of central RO (I really loved this place!).
Romania is well worth it!
In Southern Serbia, explore the mountain passe roads to ski resorts north-east of Novi Pazar (a majority muslim town that may well change your perception of Serbia...), by the Kopaonik mountains. Had much fun riding around there!
Also, as a bigger city, I found Niš to be an excellent stopover.
If your journey gets you into this area, try and follow the Serb Danube river side on route 34 (up from 33 and 165 Topolnica), from the Danube Iron Gate up to the Serb-Romanian river dam - very scenic, no traffic whatsoever (take care to fuel up well before, it nearly ran dry, not very inhabited up there!).
The small SRB-RO border post on the danube Tito-Ceaușescu dam is also much less hassle and waiting time than the other borders!
Can't comment on Hungary, it's one of the very few countries I have been boycotting up to now, partly due to their complete and utter lack of mountain roads...
Have fun!