The DX 50s seem to run without a lot of histrionics. Haven't played with the current one, when I looked at it, it looked as though they had kind of cheaped it up relative to the original. Perhaps someone who has one will speak out on it?
(warning: long post)
One of my stations has a 3DX50. I haven't been inside it a lot, and I'm not terribly familiar with it yet (after a couple of years after inheriting it). It does have its quirks.
I installed a DX-25 at an earlier job, and the 3DX has some similarities, but it seems way more complex. The RF modules are quite a bit different. Seems like the mosfets are bigger, but they are easy enough to change...I've gone through about 8-10 in two years.
The guys who installed the rig before I got there had all the fun with unstable operation. I don't know exactly what the problems were (not much was documented), but I'm guessing much of the trouble was fixed with firmware upgrades. There is a LOT of microprocessor control in the box. Open it up and there are cat-5 runs everywhere. One of the initial problems was caused by RF getting into the cat-5, or so I've heard...this was fixed by rerouting and shortening cables, and probably some judicious application of ferrite beads.
As I said, the box has quirks. The first time I stood in front of it after the switch to night pattern, the power output was swingin' back and forth by about five kilowatts. I eventually traced that problem to the "max power" parameter being set too low. Every time it hit the limit, the controller would drop the power down, then it would raise it back to the preset level, until it hit the limit again.
There is one really stinky, obnoxious, oddball thing our 3DX will do. The box is rather sensitive to power glitches (or maybe it just doesn't like the trains passing by a few hundred yards away several times daily) and it will "fault" RF modules off for no apparent reason sometimes. Every once in a while, it will "fault" an entire big-step module combination off-line. When this happens, the transmitter will still make power, but there aren't enough modules to properly generate the envelope...and the result is some really ugly splattering up and down the dial. Fortunately this is an easy condition to correct, by hitting the reset function, which can be done remotely, and doesn't even drop carrier when you do it.
I've talked to the techs at Harris about that one, but apparently nobody else has seen one do that.
Other than that issue, the box is more or less behaving itself. Lately it's been awhile since any fault lights have popped on. I've had other transmitters that would work fine for months, and then become problem children for no apparent reason. I'm keeping my fingers crossed on this guy.
If I had to buy another 50K transmitter today, I'd probably look pretty hard at the Nautel. I had a 1kW Nautel at my last job, and I never had to do ANYTHING to it in seven years except vacuum it out once or twice a year. The only time it ever went off was during a power failure (once) and when the jocks accidentally turned it off a couple of times.