With the proper tools, knowledge, and manuals/schematics plus help from the community they are excellent trucks. The A1 and up trucks are statistically THE MOST RELIABLE vehicles the US military has ever owned. And not by a little bit either - by HUGE amounts. Even the A0 trucks were 4x more reliable than a HMMWV (which admittedly were pretty terrible), and over 2x more reliable than the old M44A3 5 ton. Then the A1R more than doubled that advantage - more than 10 TIMES as reliable as the HMMWV:
Yes they have more electrical systems, but all the CAT engines used in these trucks have an excellent reputation as reliable workhorses in trucks, RVs, and school busses. The Allisson transmissions are for lack of a better term - bulletproof. You almost never read of problems on this forum or others.
The electrical that scares people really isn't that bad compared to any consumer vehicle made in the last 20 years. MUCH simpler in fact. And better documented - wiring that has number tags - pretty decent water tight connections - and not nearly as much "intellectual property".
Is it an M35 with a multi-fuel that Cletus can fix with a crowbar and a crescent wrench? No it's not that. And it will do things the M35 can't do - like climb a mountain at more than 20 mph, and do it with comfort and a cab you can hold a conversation in - AC if you want it, really good heat, etc.
The FMTV's are built with a LOT more off-the-shelf heavy duty truck parts than the older trucks. Meaning you can find parts easier and cheaper.
As with anything there is a learning curve - and if you want the advantages of the platform then learning a little bit about electrons and possibly some J1939 interfaces is a requirement IMO. Really the CAT engine computer and the Allison transmission computer are the easy ones. I have the specification and am working on an interface for the voltage regulator..... about the only thing I'm not real sure about would be communicating with the Medallion gauge cluster control but it's the same basic computer that's used on ski boats so I'm guessing there's something out there for it - at any rate if it goes out it won't stop the truck anyway.
Literally ALL the issues I've run into were due to aging rubber, corrosion from sitting, deferred maintenance, and PVT Snuffy. All things you will have EVEN MORE OF with an older platform.