What's different in the US from the rest of the world that's been running 5G for a while?
The FAA's concern here is that the bands will be neighbouring in the USA, which means if either the 5G equipment accidentally transmits a bit of signal outside its allocated band, or the altimeters accidentally interpret out-of-band signals they ought to ignore, then problems could happen.
Since radar altimeters are pretty old tech, and very analogue, their concern isn't totally unwarranted.
So, provided all hardware out there went through the proper FCC tests, and nobody cheated, then it's guaranteed to work.
The issue is that what the FCC rules say, and how devices in the field actually behave, can differ...
That's the point of the situation. You replace the radar alt on aircraft #2153 and you do not trust it until its got a few hours and thats "ok". There are published minimums and a safe pilot will not push those with something that was just bolted together.
The exciting part of 5G is its like every radar alt in every aircraft flying in the entire country just got replaced and can't be trusted until proven otherwise for a week or two. No biggie. I suspect some will indeed fail and be replaced and that'll be OK because nobody will be trusting their radar alt for the first week 5G is turned on. And after that it'll be "fine".
Its kind of like the effect of EMP on cars. Disaster doomers like the mythical story of one "boom" and all cars stop, however comically unrealistic that is. Reality is you could still cause chaos logistically by hitting just 0.1% of all cars because our supply chain, even without covid, is absolute trash. And that's how this 5G/radar situation will turn out. Most likely the supply chain for avionics literally can't handle replacing, say, 0.1% of all radar alts currently installed in aircraft today. Can aircraft spruce and specialty literally ship 500 radar alts on monday if someone snapped their fingers and turned 5G on? Are there enough bored A+P mechanics to install all of them or will aircraft sit waiting for parts to arrive or get installed? Hilarious to consider a fixed base operator trying to order ten radar alts shipped by air priority and the air priority is down, so we're waiting on trucks. Then just like toilet paper or microcontrollers during covid, you'll have idiots panic buying two year supplies "because there's a shortage" resulting in plenty of people needing parts and having zero access. So if your average airline needs ten replacement radar alts per year and the supply chain might be F-d for two years they need to panic order twenty radar alts and stick them in closets (and order that RIGHT NOW like today...); they may only need three when 5G is turned up and then seventeen other aircraft will have to sit on the ground waiting for parts because "their" radar alt is sitting in someones hoard/warehouse.
This is a sample notam for what the anti-interference alert will look like.