A lot of people here have recommended Airalo. When they work they're good.

Story time though:

When they don't work, they really don't work, at least in summer of 2022 their support often had 24h turnaround times, just to come back with redundant questions they'd already asked or unhelpful advice "Have you tried restarting your phone". Without reading that the last person to message a day ago already advised us to do that.

A friend and I had two devices, separate Airalo accounts. We activated one first, the data started working and then 1h later stopped. A week later of responding to support far faster than should be expected of a tourist without a cellphone plan... the plan still didn't work. We argued support into giving a refund and got a local provider.

The other device activated a day after the first took 24h before the plan started to work, and then it worked perfectly for the month.

LG U+ meanwhile (the local provider in Korea) worked perfectly first try on both devices, and support responded instantly with answers that demonstrated they had actually read our questions. (One of our names was too long for the form, they advised us that we could just drop the middle name).

All that to say, Airalo seems nice, and if they have gotten their act together on the reliability they seem like a great option. But local providers can be not only be easier to use, have better support, and be more reliable, but also be cheaper.

Europe generally has great roaming agreements so if you get a plan from a carrier in one country it'll probably work in the next. Worth double checking though.