IMO, it depends which bike. Among big 650 singles...we're spoilt for choice. IMO, all are pretty good. In terms of pure longevity I would rate the 650 class something like this:

1. Suzuki DR650SE
2. KLR650 / XT660R (tie)
3. XR650L
4. BMW F650/G650 series (F, Dakar, G)
5. KTM 640/690 et al
6. Husqvarna 610/630/ Terra 650

I've owned the DR, XR-L and the KLR and owned two KTM 640's some years ago. None of my bikes ever broke down but lots of friends ride these same bikes ... so I've benefited from riding with them ... and have towed a few home.

There is a lot of "unknown" when you're talking high mileage used bikes.
It's really hard to know how hard they were used and how frequently they were serviced. A loaded ADV bike ridden fast off road ages quickly. Try to look for tell tales of this wear ... or hope for a forthright an honest seller.

On high mileage bikes lots of things wear out over time and hard use. Most buyers never give them a thought ... but things like Wheel bearings, Swingarm bearings and steering head bearings, cracked frame or rear sub frame ... ALL can be worn out or broken. On any high mileage bike I would carefully check them ... no matter the brand of bike. Remove seat and tank, side panels ... put an expert eye on the bike ... or PAY and expert to do it for you.

The Japanese have a clear advantage with electrics. BMW have the poorest showing. Some of the above bikes can have cracked, bent or broken frames or rear subframes. The KLR, XR-L Honda and BMW all have a history of this ... especially if overloaded. I've been there, seen bad results in person on all three of the these bikes.

The KTM's and Husqvarnas are so rare that it's hard to collect data, especially the Husqvarnas. Almost no one is traveling on them.

We don't see many big Yamaha singles in the USA, Mexico, Cent. America or S. America. But I've read enough Ride reports from riders in EU, Russia and Mongolia to be convinced they are as good or better than all Yamaha's that have come before. So top of the heap for sure.

KLR's have a few issues that need addressing. Once done, pretty tough bike and cheap as chips to buy here in the USA.
The DR650 is, IMO, the longest lasting and least problematic. Also a pure bargain used in USA. But it does have ONE fatal flaw that happens once in a great while. 3rd gear failure which can ruin the whole motor. But hundreds of very high mileage DR650's are out there, rolling along ... including my bike at 55,000 miles. Not ONE problem.

Everyone has their favorite bikes and swear by them. Tons of happy BMW riders out there who've never had a problem ... but they DO wear out, just like all bikes do over time and hard use.

Read the ride reports, ask long time owners to learn the true history of issues. Read owner forums. The new Chinese made G650 model BMW IMO, are the best yet. But very few out there with high miles at this point.

I would buy the NEWEST and lowest mile bike you can afford. Even a low miles bike can be worn out. After ten years things like bearings, rubber lines and hoses, brake system internals ... all can begin to age. Wiring looms can be abraded on older bikes ... once it rubs through ... problems!

Unskilled owners can also spell an early doom for a perfectly good bike. So, roll of the dice. Think positive ... all will go your way! (I hope!)