Whether you personally "loathe" the show is not what is being discussed. Many music and theatre critics cite SWEENEY TODD as Sondheim's masterpiece..his PORGY AND BESS.
However you are correct to note that it is not universally adored. In the original run a feature story about Angela Lansbury noted that getting people into the thetare to see this show was a huge challenge. While many did end up loving it, others were put ff by the violence, the jolting factory whistle and the broad performances. Many people left at intermission. The awards, the critical acclaim and snobbish attitude that anyone who didn't like the show was not culturally connected.
For me it was a genuinely thrilling show, but one that does not appeal to everyone. Even so, I believe it has earned it's place on the list of modern musical masterpieces. Along with SHOW BOAT, PORGY AND BESS, OKLAHOMA!, GUYS AND DOLLS, MY FAIR LADY, WEST SIDE STORY, and GYPSY. Each of these shows has flaws, but they stand up beautifully to repeated viewings and that is a crucial criteria for a "masterpiece."
Cast albums are NOT "soundtracks."
Live theatre does not use a "soundtrack." If it did, it wouldn't be live theatre!
I host a weekly one-hour radio program featuring cast album selections as well as songs by cabaret, jazz and theatre artists. The program, FRONT ROW CENTRE is heard Sundays 9 to 10 am and also Saturdays from 8 to 9 am (eastern times) on www.proudfm.com