Two long-distance trails are (theoretically) traversing the length of Italy: Sentiero Italy with a total of almost 7,000 km is spanning the entire alpine range, the length of Italy in several variants plus Sicily and Sardinia and is maintained by CAI. The E1 on the other hand shortcuts the alpine range by traversing the Po Valley, has no variants and currently ends in Fortino, although there is already E1 trail in Sicily. It is maintained by the FIE. Both trails use existing paths and often run parallel or close to each other. Where they use popular trails like the Alta Via dei Monti Ligurii or the Grande Escursione Appenninica they are well marked, where they are "stand-alone" their state ranges from brand new fabulous trail to non-existant.
Generally speaking it is European idiocy that there are two separate long-distance trails in Italy and none is "finished" or even close to finished. Instead of joining forces the CAI and FIE are pursuing different plans just because of one fact: Only the FIE is a member of the European Ramblers' Association which organanzises the European long-distance trails. Sentiero Italia cannot be part of the European network because CAI is not a member of the head organization.
The Sentiero Italia has already been founded in the 90s but fallen into decay. Only since 2018 the CAI has been reviving it. But instead of now putting all the efforts into really "finishing" the Sentiero Italia the CAI has now announced that it will create a new trail linking all the National Parks in the country. With limited resources I am afraid that this fact will threaten the fragil state of the Sentiero Italia again.
Generally speaking I found the E1 far less developed than the Sentiero Italia. I would not recommend the E1 because it is impossible to thruhike it in Italy! The E1 just does not exist in places and has never existed there other than on paper. One example: At one place the E1 is supposed to cross a broad fast flowing river where there is no bridge or ferry! Only the FIE knows why they routed the trail there ... To find an E1 marker in the mountains is also a rare occasion whereas the Sentiero Italia is almost 90% more or less marked - which does not mean you can hike it there. Often the trail keeper has been the last person through here - sometimes several years ago judging from the state of overgrownness ...
The good new is that there is enough water, at least for thruhiker standards. You don't have to carry more than for one day - if you knew where the water sources are!!!! And that is the biggest problem: OSM maps don't show most of the sources and the ones that are shown can often be dry or turned off. Unfortunately neither CAI nor Vasentiero mention water sources on their website probably assuming that no one thruhikes the trail wild camping style. But even if you stayed in paid accommodation every night it would great to know where each section's water sources are. One good thing is that almost every little village has a public water fountain and on most mountain passes there is a guesthouse, mostly with accessible water taps.