We just returned from a few days in Paris and London. At one point when I was booking Eurail travel I purchased a data SIM from their vendor. It took forever to arrive and didn’t seem to work quite right. Then I did a bunch more SIM research–there are a lot of options out there. But in the end we decided to pony up $10/day for the Verizon deal that let’s you use your normal voice and data plan internationally. And I can say that we found the convenience and consistency of using our normal phone plans was worth the price!
Also, there’s the issue of needing to receive calls and texts on our normal numbers–particularly since I had used my cell number as the contact for travel arrangements. The downside, of course, was getting spam calls as usual. Had to shut the phone at night due to the time difference.
FWIW, 4G coverage in Paris (Orange) was excellent, even in the Metro. In London service was variable and sometimes strangely slow even with 4G and solid reception. No service at all in the London Underground.