Synthesis received generally positive reviews. At
Metacritic, which applies a
weighted average to reviews from mainstream critics, the album was given a score of 69 based on four reviews, indicating "generally favorable reviews".
Renowned for Sounds Rachael Scarsbrook regarded it as Evanescence's "most ambitious project to date", writing, "there is a beating heart and spirit within this release that still hurts and evokes the most cathartic of responses". Suzy Exposito of
Rolling Stone said that the remake of "Bring Me to Life" is an "act of artistic justice" for Lee and highlighted the album's two new "ambitious" songs. Kerri-Ann Roper of
Belfast Telegraph felt Lee's vocal performance is "as powerful as ever" and
Synthesis "is proof that old can be the, well, new new and still be just as satisfying." Chad Bowar of
Loudwire considered the album "dynamic and compelling", and praised Lee's vocals, the mix of various instruments and the production, describing it as "grandiose and bombastic in parts, quiet and subdued in others". Christa Titus of
Billboard called
Synthesis a "risk" and said the "elegant orchestrations" enhance "aspects of [Evanescence's] music that are often ignored". Siena Yates of
The New Zealand Herald complimented Lee's voice and the "cinematic" classical foundation of
Synthesis but found the album "weird" and "largely unnecessary", believing that stripping down and reworking the hits removed the "nostalgia" and "hooks" that made Evanescence stand out in their early days. Writing for
Classic Rock, Stephen Dalton said that the "guitar-free remakes have a windswept grandeur and widescreen sonic palette lacking in the original recordings", and complimented the new songs, dubbing the album a "successful experiment".
Blabbermouth.net deemed it Evanescence's "most ambitious effort to date", writing that "Lee's incredible vocal range and the group's expressive, multi-layered music naturally lends itself to orchestral accompaniment", creating a "truly dynamic, cinematic sound" while the passion "undoubtedly shines through".
Beat Magazine hailed it a "riveting" album that takes the listener on a "sensory journey", with vocals that convey "piercing emotion".
Varietys Roy Trakin wrote that "Lee firmly re-establishes herself as one of rock's pre-eminent vocalists", while the songs become "full-throttle, wide-screen epics" and the album evolves Evanescence's sound. Catherine Morris of
Metal Hammer considered
Synthesis "an ambitious and unexpected move", stating that it impels Evanescence's music "into a new, alternate dimension", giving songs an "ethereal, cinematic makeover" and drawing the listener's attention to the "core melodies". Jim Fusilli of
The Wall Street Journal felt that "the shift in style isn't without its risks" and "for the most part, the new approach works well" while the electronic percussion "gives the music yet another new trait". Reviewing for
Kerrang!, Paul Travers expressed that the orchestral elements aren't a radical transformation as Evanescence's rock music "works with space and texture and [is] built around Lee's vocal and piano". Lee's vocals are "more confident" and the album "changes the format if not the feel" of songs, but the "electronic embellishments" felt distracting.
AllMusic's Stephen Thomas Erlewine called
Synthesis a "successful fusion" that "amounts to a step forward" for Evanescence, regarding it a "natural fit" for Lee, whose "powerhouse vocals often wrestle the orchestra into submission", and the "layered, skittering electronic rhythms" help give it "a steel spine". Alan Sculley of
The Morning Call deemed it an "ambitious" project. In
Music Week, George Graner characterized the album as inspired and a "bold artistic statement". Writing for
Gig Wise, Catherine Verrechia said that introducing an orchestra feels like a "natural progression", and "surprisingly, the music still has many doses of heavy, atmospheric aspects". Evanescence "have progressed to another level musically" and
Synthesis "is a prime example of showing growth". ==Commercial performance==