Iwasaki specifies the use of constant-wattage auto-transformer (CWA) ballasts without ignitor for this particular lamp, which is a typical practice with standard probe-start sodium-scandium iodide lamps intended for the North-American market (your lamp should have an E39 end cap). Given that, their restriction about mercury ballasts certainly stems from their low open circuit voltage. The ANSI M58 CWA ballast suitable for your MF250X/U (specifically mentioned in Iwasaki's datasheets) has a nominal OCV of 305 V, so it makes sense that the lamp doesn't start properly at 250 V (that's the reason why dedicated transformer ballasts were introduced in the USA when metal halide lamps were first released there around the mid 1960s).

Now, on the possibility of running this lamp in 220-250 V countries using a series choke ballast, you certainly can use an ignitor to start it but there are some limitations however: only a soft (700-1000 V pk) parallel ignitor may be used since the lamp was not designed to take multi-kV pulses (which can cause external arcing between the burner leads if the discharge fails to ignite while cold), and the low ballast OCV (equal to the mains voltage here) will affect the glow-to-arc transition negatively, thus causing a more rapid degradation/blackening of the burner over time than on the high-OCV M58 ballast. Overall, using a standard European MH ballast with 3-5 kV ignitor does not pose any significant problems as long as you're only testing a brand new NA-type probe-start 250 W MH lamp, but you'll run into some potentially serious troubles if you want to put that lamp into regular service using such setup.

Sodium-scandium MH lamps sold in Europe for a use on standard HPMV reactor ballasts with an additional igniter are modified versions of their North-American counterparts, they are not interchangeable although they share similar steady-state electrical characteristics.

Thanks for the information Max. It is very unlikely I'd ever put this lamp into regular service. Very few of my lamps do a real nights work. Some do get tortured a bit though

So, all is good then