I bought an abused 220k Ecoboost wagon during covid, did a little camper build, and have since put about 60k on it.
The main issue with the Ecoboost is exhaust manifolds warping and breaking bolts which seems to happen anywhere from 50k to 150k. Access is not as good as in the trucks, so it's long and tedious but not that hard if you have some strategy for extracting broken bolts. The factory fixed this sometime in 2021. I think it's also worse in the vans because water drops from the base of the windshield onto the turbos. This is also the source of some other possible issues, like wet air filter, bad main electrical harness connections, and things on top of the engine like coil bolts that don't come out because water pools on them. Easy enough to prevent with any number of ugly homebrew water diverters, but something to watch for. (Various ziploc bags and iced tea bottles underhood might actually be a positive, showing the owner was aware of all this and proactive about it.)
Both the EB and the 3.7 seem relatively reliable and multiple hundreds of thousands of miles doesn't seem rare; the 3.7 obviously doesn't have the turbo and manifold to go bad, but does seem like it more frequently needs cats.
The diesel is rather problematic. For emissions systems mostly I believe, but unlike the trucks the knowledge base just isn't there and the engine was only used in the Transit in North America, so I would avoid that orphan. The cheapest used Transits tend to be diesels; that should tell you something.
If you're in the rust belt watch the front subframe for rust. There are a couple of bolts that are a bear to get out to replace the front control arms or subframe, and it ends up all needing to be cut into pieces if you need to, for example, drop the trans pan. Transits seem to rust from the bottom up, but it doesn't look like any of the modern vans are particularly corrosion resistant.
Other things to be aware of on the platform: tire size is limited to about a 245/75-16 and the speedo can't be fully corrected to that easily. Ground clearance and particularly departure angle on an extended van are worse than most alternatives. Passenger vans have the world's biggest side airbags that go all the way back and can't be disabled without also disabling the front row side bags, if you're looking to build stuff in the rear. The rear end is a full-float 9.75 and rear brakes go fast (like 30k) and can be expensive if you were to pay someone (But it also means that an F-150 eLocker should bolt right in). As has been stated, cooling is probably the towing limitation; having a rear air conditioner drops the tow rating, for example.
They're a weird mishmash of Euro Ford and US Ford, resulting in stuff like the aforementioned windshield dumping water onto the engine or needing to take the subframe out to take off the transmission pan but in general they're pretty straightforward to work on and parts and info are pretty accessible.
Transits do drive better than Sprinters or Promasters, and are in a different league from Econolines etc. especially in GRM qualities like steering and braking. A Transit would still be my choice of van, particularly now that prices are coming down a bit.
