Haven't tried Fareboom, but for multi-city in Europe I've had good luck with Tripmasters. I usually call them and customize it, e.g. you can fly into one place, not get a hotel there and go on your own if preferred and just place hotels here and there where you want them (usually 4 nights minimum for Europe scattered here and there -I usually might just get first night or last night before flying out). Last year we flew into Edinburgh, on to Moscow, then back home from St. Petersburg with 4 nights hotels (3 weeks on our own), for about half of what that just multi-city airfare priced out at on most websites.
Good to know! We typically do Airbnb's and have found some great deals, but I always like to explore all options! We don't necessarily need a multi-city, but it's not much more vs just doing Rome round trip. Especially considering we would need to take an additional train from Rome up to Milan if we did do RT Rome.