I have crossed a couple of dozen land borders, some of which took four minutes and one of which took most of a day. The variation is enormous and the things that determine which kind you get are mostly knowable in advance.
The information problem
Land crossings are poorly documented compared with airports. Opening hours change, procedures change, and a crossing that was straightforward last year may not be.
Official sources are the only reliable ones and they are frequently out of date. Forums and recent traveller accounts are more current and less reliable.
What is worth confirming before setting out: that the crossing is open to foreigners, that it is open to your nationality, its hours, and whether visas are issued there or must be obtained in advance.
That last one has stranded people I know. Some crossings issue visas on arrival and some do not, and the same country may do one at an airport and the other at a land border.
Vehicles complicate everything
Crossing on foot or by bus is generally simple. Taking a vehicle is a separate process with its own paperwork.
What is typically required: proof of ownership or a hire agreement explicitly permitting the crossing, insurance valid in the destination country, and sometimes a temporary import document for the vehicle.
Hire cars are the common failure. Most rental agreements prohibit crossing borders entirely, or permit it only with prior arrangement and an additional fee and documentation. Turning up at a border with a car you are not permitted to take across ends the journey.
Insurance is the other one. Domestic policies frequently do not extend, and cover must be bought, sometimes at the border and sometimes only in advance.
Time of day and day of week
The largest controllable variable.
Borders have rush hours, driven by commercial traffic and by local movement. Early morning is generally quiet at most crossings. Late afternoon on a Friday is generally not.
Public holidays in either country change everything, in both directions — sometimes closure, sometimes enormous queues from people travelling.
And commercial traffic queues separately at most crossings, but not all, and where it does not the wait can be hours behind lorries.
What actually happens
The sequence is usually: exit formalities on one side, a neutral stretch, entry formalities on the other. Both are required and people occasionally miss the first, which causes problems later.
Expect to be asked where you are going, where you are staying, how long, and how you are funding it. Having specific answers, including an address, makes this shorter.
Onward travel evidence is requested at some crossings and not others, and having something to show is easier than arguing about it.
Vehicle searches happen and are routine. They take time and are not usually a problem unless something is genuinely wrong.
The one that took a day
A crossing where the system was offline, the queue was long, and nobody was giving information.
What made it survivable: water and food in the car, a full tank of fuel, cash in a currency accepted on both sides, and the fact that we had not booked anything for that night that could not be moved.
What I got wrong: arriving in the afternoon, which meant the wait ran toward closing time, and having nothing offline to read or navigate with once the phone lost signal.
Money and the informal economy at borders
Border areas have their own arrangements and they are worth being cautious about.
People offering currency exchange, assistance with paperwork, or expedited processing operate at most busy crossings. Some are legitimate and some are not, and the rates offered by informal changers are usually poor.
Fees should be official, receipted, and posted. Requests for payment without a receipt are worth questioning politely, and the answer varies by place.
Carrying some local currency for both countries in small denominations makes everything easier, since border facilities frequently do not take cards.
What I do now
Confirm the crossing's status from an official source within a week of travelling.
Arrive early in the day, and not on a Friday or a holiday.
Carry printed copies of everything — insurance, hire agreement, ownership documents, bookings — because phone signal at borders is unreliable and officials generally prefer paper.
Have cash in both currencies and a full tank.
Do not book anything for the arrival night that cannot be changed.
And treat the crossing as the day's activity rather than as something on the way to somewhere, which removes the time pressure that makes all of it worse.
Buses and shared transport
Worth noting because it is often the better option.
International buses cross borders constantly and the drivers know the procedure. Passengers are generally processed as a group, which is faster, and somebody who does it daily is handling the paperwork.
The trade-off is waiting for the whole bus, including anybody who has a problem, which occasionally means an hour standing in a car park.
For a crossing with a reputation for difficulty, going by bus rather than by car removes most of the variables, and hiring a vehicle on the other side is frequently cheaper than taking one across.