How much of an issue are they? The one we are looking at is a crescent and is fairly long. Probably 100 houses. It is not in a bad state of repair no potholes and pretty smooth. No pavements but there is street lighting. Not sure hoe repairs are dealt with so will ask the vendor.
Normally you pay a decided amount into a fund, managed by some management company or one of the residents. Unadopted roads can mean huge expenses ( drains, road surfacing etc) some time or the other. You still pay council tax but don't get any benefit from it!
We pulled out of a sale of a house on an unadapted road when our lawyer told us what the ramifications were. It was years ago - can't remember exactly, but we were young and poor and just couldn't afford to take the risk.
If its a new development, sometimes the council won't adopt the road if there is a planning application in to build on a neighboring plot.
So if there is any empty/re-develop-able land around the site i'd be very wary as the unadopted road could end up being used for access to the building site.
In my experience - the road fund usually comes up short, one of the numpty neighbors starts campaigning for a gated community (to keep the hun at bay presumably ) - and then another starts parking their fleet of work vans tying up parking spaces.
The only people who benefit are developers as they don't have to bring the road up to a certain standard that a council would insist on before leaving the development.
There is also road marking, street signage, speed control to consider.
I would contact the highways department and ask if they are aware of any issues (to see if have tried to get the road adopted but failed).
Don't buy a house on an unadopted road. It is a nightmare and if it is a long road, even more so. Potential problems are potholes, damage to cars, how repairs are paid for (there will be people unwilling to pay). Then when the road is damaged, you will find that taxi drivers, relatives etc won't want to come down the road.
Find yourself a nice property on an adopted road. That is the best advice I can give you.