"The islands" is pretty broad! I would consider Crete in a different category from the islands in general, more similar to mainland Greece. It is geographically an island, but a large one, with a year-round population over 600,000. Infrastructure in the cities is comparable to mainland cities. Heraklion has about 200,000 residents and has the services you'd expect for a city of 200,000.

Many of the Aegean islands are a different story. Many have small, elderly, and dwindling year-round populations and economies that survive almost exclusively on summer tourism. Infrastructure and transportation varies quite a lot, especially in the off-season, and especially on the less populated islands. Although nowadays internet is pretty decent in the main towns. I'm not personally familiar with Corfu to say anything about it.