this is news to me, had my Roco crocodile since 1980 (AC version-analog) and another one since 1990, run them together with a heavy load behind them and never, never, NEVER had a problem.
any loco (doesn't make any difference which brand you are talking about) which isn't aligned properly will suffer and the motor will go hot.
There is a problem if you take the screws off (for instance changing rubber tyres) and you don't put the screws back where they came from, (the are slightly different in lenght) and by flipping them from one wheel to another can cause the loco to stop.
So if you have experienced this with your connecting rods, the engine warmed and finally failed you may look into this more closely and you may will find there is nothing wrong with the loco's design or connecting rods.

By the way, the Roco model does outpull the Märklin model, so in this instance, all metal doesn't always work when it comes how many carriages can it pull.

the advantage of a Märklin crocodile is: you can add a sound decoder and it has prototypical light change but I think the Roco crocodile has the same now with its latest model.
another difference is the size, the Märklin model is bigger in appearance: wider and longer and has a different axle arrangement

John

You should ask some Swiss enthusiasts who were there as the first batches came onto the market.....THis is wellknown, and it took quite some time before Roco succeed in correcting the defaults....(still today we can read forums on which people got troubles with some of those Roco models).
About the pictures of the Marklin Crocodile : you should give the reference of your model (there were at least 3 different molds ....Yours doesn't seem to be the most recent one. Difficult to say on pictures, but It looks like the old 1/76 scale...One thing I'm sure about : the last references 39xxx are different)
thank you.