You didn't say WHY you would recommend against it.
Improper and or questionable fitment. And I've had friends use 'wobble' bolts before only later to have issues with a lugnut (or two) shear off completely.
I've dealt with tire rack when it was magazine order only..(and their featured wheel was the mini truck "hammer" style wheel... 1989 what!) They actually sold ICW wheels....oooof!
I know they take their wheel fitment and associated hardware very seriously. I've had them flat out refuse to sell me certain wheels I knew would fit (I had friends who had them) because it wasn't listed for my car and they didn't have the chance to test them on that model.
They would open themselves up to a huge liability law suit if they put anyone in harms way or sold a questionable product. It's impossible to know if they have or haven't sold these types of lugs because they do not list them on their site. I don't blame them for not listing them either. There's a huge "this doesn't sound safe" contingent that would never order anything. If you have any doubts, call them on the phone and talk to the technical specialists. They don't care if you buy a 4X100 wheel or a 4X98 wheel (they sell both) Buy the style, correct offset, and correct hub bore size and they will get you what you need to insure fitment.
4X98 and 4X100 is .5mm difference per lug.
I'd have way more reservation using tuner lugs or spacers..and people don't seem to mind those nearly as much.
I personally don't care if you decide 4X98 is better or 4X100 is better. I'm just saying that I have zero issue going 4X100 with movable conical seat lugs. I've used them long before Fiat was ever sold in the states. If the situation calls for it, I'll use them long after this car is out of my garage and is someone else's car.
'Tuner' lugnuts are just wheel locks for all 16, 20, 24, or 32 of your studs. Spacers/adapters are made to be made to be hubcentric (if it isn't a hubcentric spacer, that is what you don't use). The other issue is the potential increase in improper torque values on your studs. Just because you haven't had issues before doesn't mean it is ok. And Fiat had sold cars in the United States prior to 1987.
I know Tire Rack more than most are very careful about what they recommend. And there have been more than a few instances where they've given me disclaimers about things because they aren't listed on their website, but they work perfectly fine. The interesting thing is that they have been so careful on so many things, but not here? Why that is is beyond me...
If they are not doing the same thing with wheels that fit General Motors vehicles (most being 5x115), to fit Ford cars (typically 5x114.3), than why not? It would be FAR LESS difference than the 4x100 on a 4x98 Fiat 500. That said, the most important thing as always is to stay safe, and thoroughly enjoy your Fiat 500s!