I recently purchased the ESU 31033. BR 151 in chromedioxide green.
I've been very happy with it and would say the cost is absolutely worth it for what you get.
1) It is a loco that can easily be switched from 3-rail to 2-rail. I recently saw a video on a
youtube channel where a marklin guy took his ESU loco over to someone that runs 2 rail DCC and just pulled the slider (which switches the loco's pickup connections to 2 rail) and ran it on the 2 rail DCC system. No issues.
2) I've had no issues running mine - but I've not run it on a layout yet, just tested it, so if there are contact issues internally with the mechanical switching contacts of the slider, I don't know. I also don't plan to remove the slider (which comes preinstalled) so maybe some of the internal switchcing from 2 contact to 3 contact only happen when one uses that feature several times in removing the slider, running in 2 rail mode, then plugging it back in to run in 3 rail mode, etc.
3) The loco is very solid built and heavy - so its got serious materials in it -not plastic all around, and that costs money to make. The detail on it is also very good (by my standards at least).
4) The added features such as the internal machine room sparking LED, or the brake spark LED features, or the wheel squeel in turns sensor and sounds, or the going over turnouts sensor and sounds, these are very cool features I'm glad it has, so I can do freight operations and get more of the realistic type effects.
The video I put on my channel is just the sample of the features I tested when I first got the loco this past year.
As far as I can tell, though I've not researched it deeply, is there is no
mess, and suggesting it is hyperbole.
I would say that searching Stummi for "ESU 31033" results in only 7 posts, and no thread on serious problems with the contacts. If there was a
mess, as suggested, then I'd expect there to be a thread or two complaining about the quality of ESU locomotive functionality, and there isn't any. So I've got no support for this notion that there is an issue, much less a mess, other than some person(s) in a store may have complained, which is anecdotal evidence being such a small sample size, and meaningless to draw conclusions from.