I've seen some decent prices on used wheels built on Power Taps, on Ebay, and I wonder if these are a good idea.

Overall quality as a hub? I've been using a 2006 Campagnolo Record hub.
Quality of the bearings?
Maintaining bearings?
Are there any non-recoverable failures?
Is there a cadence sensing feature? What models?
compatible with Wahoo Elemnt?
Can a Campy-compatible freehub be installed?
Do they have any constraints on lacing? I'd swap a 32 hole Mavic GP4 tubular rim over to it.

The bike's a bit vintage, but ... I'm fine with that. It's a 1984-ish Mondonico, now running Campy 2x10.

I have a PowerTap G3 (will probably be selling it soon to), it broadcasts on ant+ and bluetooth, mostly survived multiple crashes (even sliced part of it off in a crash), SARIS is AWESOME when it comes to their warranty (they got it working fast and free of charge). It's very accurate and auto zeros when you coast, the only issue is it's built into your wheel making it hard to upgrade if you want to in the future.

If you get a G3 hub (3rd generation), the bearings are sealed; older models may have free ball bearings with cups and cones -- you don't want those.

All PT hub models sense speed and cadence (though the cadence can be fooled, it's usually pretty good).

The current G3 models broadcast on both ANT+ and BT, so they're compatible with almost everything. This might not have been so for previous generation hubs -- you don't want those.

For the G3 models, you can get a campy freehub.

They're a mature product that generates reliable high quality data. Just keep to the current generation (G3).

No. For Gen 2 and earlier, the problem was with the cones: they were soft, so wore unevenly and produced unacceptable amounts of play. People used to buy Dura-Ace cones and grind off a bit to make them fit. That was a pain in the butt.

In addition, the electronics in the G3 hubs is more localized to the end cap, so if anything wrong happens there, you can just mail back the cap rather than the whole wheel.

I may have OP confused with another poster but...are you not the guy who is riding on platform pedals? If so, positioning of your feet on the pedals and securing them will have a yield both from performance and safety i.e. feet not slipping off the pedals during a sprint and descending. As far as tip over issues, that is a matter of practice must like learning to ride on two wheels only much easier.

The first PM I got was a power tap and I agree with the others with respect to performance and reliability but the moment I put the PT on the bike and started to get numbers, I regretted not getting a crank based power meter.

If you need pedals and want a PM, there are a lot of good power pedal alternatives and pedals can be moved from bike to bike and different wheels can be swapped on the bike and still have power. You may not need or want this type of solutions but my reason for calling your attention to this point is that there is a progression of actions that follow getting a PM. Generally, but not always, power data enlightens the cyclist and they think of new and interesting ways to use the data. A power hub embedded in a wheel has limitations that are soon realized. YMMV.