Cellular devices intended for a specific geographic market are often optimised for that market. While the majority of cellular radio bands and Cellular channel frequencies are consistent around the world, some countries have additional cellular bands that are not used elsewhere. WiFi is generally ubiquitous - although some 6 GHz WiFi bands are not available in many countries.
As such, a current Cellular model iPad purchased in the US will be "optimised" for the US will be usable within Europe, but may not cover all the available channels used by your European Cellular Carrier. In most situations, any difference in band-coverage will not significantly impact the usability of the iPad with the local Cellular Operator. Consider that Cellular devices are designed to work while travelling between countries - and/or to "roam' between networks and across national boundaries.
If making a new purchase, consider the warranty implications. Devices purchased within any EU Member State have at least a two-year warranty that will be honoured within any member state. A device purchased within the US will only have a one year limited warranty.
Also consider that there is very little actual cost differential between devices purchased in the US or the EU. US prices are exclusive of purchase taxes - which must be added to the price paid. Prices within the EU are inclusive of VAT. Also consider that you may have to pay duty upon import to the EU - to which VAT will be added.
It is generally recommended that you purchase a device in the country that you spend most of your time - as this simplifies both purchase and warranty - plus you have a device that is optimised for your country/region.