I did the drive from Crete to Poland via Macedonia and Serbia last year. It was absolutely fine and took 2 days with an overnight stop in Budapest.
Forget all the Horror stories and ignore the Armchair experts.
The Motorway between Athens and Thessaloniki is great but fairly costly, (Around 30 euros) and the road between Thessaloniki and the Macedonian border is only single carriageway but good quality. Expect a 45 minute wait to cross into Macedonia from Greece and Macedonia isn't signposted from the Greek side so you will have to follow signs for Skopje which is signposted. (The Greeks still think that Macedonia is theirs)
Another good tip is to make sure you have a full tank of fuel on entering Macedonia as Petrol although cheap is quite scarce and many fuel stations only have diesel. However you will only be in Macedonia for around 2 and a half hours.
Make sure you have your insurance Green Card which is green piece of paper to say you are covered in the country. A cover note won't do and they won't accept anything that isn't green. I had to buy insurance on the border and it was around 60 euros. Make sure you get your Passport stamped too as if you don't you may have issues entering Serbia.
Heres an idea of the queues involved,
The roads in Macedonia are reasonable quality and pretty much empty. Although a bizarre design as the carriageways can be several miles apart as though they have taken 2 A roads and made each of them one way to create a motorway of sorts.
we hardly saw any other road users passing through and had no issues whatsoever. There are toll booths but the money involved is minimal and they accept Euros and all the pricing is in Euros.
Heres a toll booth
The Macedonian/Serbian Border comes up pretty quickly and is pretty painless but again make sure you have you're green card and you get your passport stamped to say you have left Macedonian and again to show you have entered Serbia.
Serbia is a fairly unwelcoming looking place but the roads are ok and the people are friendly, the standard of driving is also pretty good in general. The road between the border and Lekskovac is a pretty twisty although well surfaced single carriage way road (Think British A Road) Make sure you have your headlights on dipped beam at all times. (I didn't see a single policeman though)
Also there are no toll roads for the Serbian leg.
Here's what the main motorway from Lekskovac looks like
Traffic generally moves at around 160 kph and this can be maintained pretty safely. Keep your tank topped up as fuel stations are sporadic.
We hit Belgrade just before dusk on the first day in the middle of a huge thunderstorm and decided to dash for the EU border of Hungary as northern Serbia although safe can be pretty overwhelming with bomb damage from the early 2000's still visible. Its also a pretty grey and dismal but maybe that's because the weather was grey and dismal when we passed through. Although on a brighter note 2 girls in bikinis washed my windscreen whilst i bought petrol
The road between Belgrade and the Hungarian border is amazing quality and arrow straight. You can do this stretch at pretty high speed but be careful near the border as the Police are everywhere on the approach to the crossing.
We waited for about 90 minutes to cross and this was at about 11PM make sure you have all your documents to hand as the border guards aren't the most approachable of people.
Back in the EU! Hungary is highly developed with excellent roads and a high standard of driving. Make sure you pull off at the first fuel station to buy a vignette which isn't a fancy sticker for your window and is simply a till receipt and your reg number entered on to the central road tolling database. Make sure you buy this as the police are hot on non payers. Stick to the speed limit and keep your dipped beam on at all times as there are police Everywhere!
I stayed overnight in Budapest, and then headed for Bratislava, No border crossings but you will need a Vignette. This time you get a pretty sticker for your windscreen! its about 12 euros for a month!
Slovakia is pretty boring however the Czech republic is quite pretty.
I'm guessing at this point you would head to Austria and then into Germany to get back to the UK (I went to Poland)
All in all its NOT dangerous, The standard of driving is far better than on Crete, Everyone i met seemed very friendly, They take euros everywhere and also credit cards.
Keep your tank topped up throughout and travel with a bit of cash too and expect delays at border crossings and you will be absolutely fine.
We are doing the same trip again in September and it really isn't a bad drive.
Good Luck and drive safe!