My roster of modern K Line and Williams is limited (4 engines total), but what I have (over 10 years old) gets very good marks from me for ruggedness and reliability ("R&R").
Regarding MTH, I have about 10 engines and I also give them very good grades for R&R with one exception. MTH Proto 1 engines need to be run at least once every few months to remain in good working order because of the battery in them. I'm aware that the battery can be changed to another kind of battery to eliminate this problem, but I have not done that yet. I think Proto 2 and 3 are very good for R&R.
Until I joined this Forum around November of last year, I would rotate my hobbies (trains, songwriting, golf, archery shooting targets with recurve bow, and reading) periodically. My wife tells people that being married to me is like being married to 5 different men! LOL. I love that. Anyway, I could easily get engrossed in another extracurricular activity and stop doing trains for years at a time, but I always ran my Proto 1 engines at least once every 6 months knowing there would be problems with them if I didn't.
I have 2 LionChief+ engines purchase since February of this year. I love the features and relative affordability, but one of them has already had its sounds repaired twice by Lionel under the warranty. For that reason, I give LionChief+ a grade of C- for R&R. To its credit, Lionel has been very good servicing it under warranty, but once the 1 year runs out, I'm only going to run it sparingly to entertain my granddaughters or when I have an audience.
I would love for Lionel to figure out a way to enhance the R&R of its LionChief+ engines, especially its sounds and the wiring, etc. related to it.
IMO, hands down, Postwar Lionel, especially its late 1940s and early 1950s up to about 1957 is King when it comes to R&R.
I greatly appreciate all of the input you folks have given regarding this topic.
Arnold