The issue with BFZ is that it's one of the worst sets that seems to ignore 90% of previous design trends coming out 20+ years after the game released. Ice Age at least had the excuse of being a learning stage, same can go for Kamigawa(though in a different way).
I don't think you can look at it purely in a vacuum, because if you do it obviously isn't the worst. But in context, the set is surrounded by good-greats sets(minus Fate Reforged), including a contender for the best designed and developed set in Magic(Khans, obviously). To go from Khans, where you had uncommons be set defining cards(Seeker, Swiftspear, Charms, etc) that were simple and clean to Dominator Drone/Mind Raker at Common and the idea of "New World Order" falls apart.
Oath redeems BFZ, but it only does so by abandoning every mechanic from it and having 0 interaction with the cards. The power level between the two sets is also startling, as Green gets good cards in Oath but shit in BFZ. Like, ignoring Shadows block(Which has issues with Escalate coming out of nowhere and being on few cards) Kaladesh has the Puzzleknots+ ways to sacrifice stuff, be it for energy or what have you, as well as a critical mass of Artifacts(Either artifacts or tokens from Fabricate), both of which work well with Revolt/Improvise. Same goes for KTK Block, where each Dragons keyword works well with the Khans. Prowess likes Rebound, Bolster+Outlast work well, Raid+Dash work well, Ferocious/Whatever Atarka's is complement each other, and Exploit can fuel delve.
BFZ deserves to be called the worst block, because it's surrounded by much better sets, abandons New World Order, has no little synergy between sets, and had a completely sucky limited environment.
Ice Age was part of the beginning of Magic, when they had no idea what they were doing really. BFZ came to us from an experienced Development/Design team, one that has none of the excuses of Ice Age's design team.