I will say that Cambridge isn't immune to problems in its quickbuilds. Look at Mass Ave, from Arlington to Harvard. The first segment from Alewife Brook to Dudley was "quick-built" in Fall 2021. At the time, the rest was to follow briskly in 2022 with more quick builds, with it complete by end of year. Instead, in April 2022 after much pushback they adopted a "partial construction" (eliminating the center median) approach - by now, construction is tentatively expected to start in Q3 2024 and be finished by Q4 2026.Quickbuilds in Cambridge are great, and they're really well done. That being said, from a technical/design perspective, they're basically identical to what Boston has done on Cambridge St NB, South Huntington, etc--just flex posts and paint, no digging or curb reconstruction. And despite this, Cambridge has been able to build a much more substantial network in the same amount of time.
The issue is less about the technical constraints and more about political will and avoiding projects getting bogged down in meetings, review, stakeholder complaints, etc (just see Charles Street from the circle to the common--perfect candidate and yet locked up in complaints from business owners along the street.
The Cambridge process, and it's Cycling Safety Ordinance, force the city to build and take action rather than delaying over and over, because it has strict timelines and success criteria. If Boston is serious about this, they must consider a similar strategy.
That said, the constructed approach seems to be a much better solution maximizing the available cross section, even if it'll take longer.