Are there really any people who think that European HID lamps are NOT compatible with US HID ballasts?

For example, are there people who think that a European 70w HPS lamp is incompatible with a US 70w M98 MH ballast even though it has the same operating voltage and current as a US 70w M98 MH lamp?

Youhave to be careful about details:
European lamps are (pretty all) designed to work on a series choke on a 230V supply. Mainly with saturated vapor lamps (e.g. HPS) that means the ballasts are purposefuly designed to slightly saturate, so the current will drop more significantly when the load voltage rises, so suppressing the thermal instability of the saturated vapor lamps. On the other hand the arc voltage of the lamp at the nominal power has to be within tight limits.
On the other hand US tends to use CWA (for HPS), which is essentially a constant current source, so needs a lamp design where the arc voltage is not that dependent on the feed power, so where e.g. the cold spot is decoupled to some extend from the direct arc heat. But if that is fulfilled, it can tolerate wider arc voltage tolerance.
So while S56 seems to be the same 87V 1.8A rating as the European 150W HPS, they may perform very poorly on the other side of the pond (cycling,...).

However as far as I know the pulse MH in the US are designed to work with HX transformers, which are essentially the step up transformer and the ballasting choke integrated into one common assembly, so a 1.8A HX with 230V OCV will have exactly the same chracteristics as a 1.8A series choke on a 230V supply, so I wont expect any problems with European lamps on these (provided the ratedcurrent matches and the ignitor peak OCV is sufficient)