Yes, as a character and the main protagonist of The Sopranos, Tony is utterly compelling, cool, awesome, etc. But from a Mafia standpoint, the guy was really probably the worst Mafia Boss ever depicted on the screen (big screen or small). He inherited from Jackie Aprile,Sr a large (for NJ standards) and prosperous family with six fully functional crews, a great partnership with NY, great rackets, etc.

Within 7 years:
He almost started a war three times (Over Tony B., over Vito, over blowing up Phil's wireroom)
Killed off the family's best earner (Ralphie)
Mistreated and underpaid valuable soldiers to the point that they turned to other sources of income, which resulted in them getting nabbed by the Feds (Pussy, Carlo Gervasi, Eugene, possibly Ray Curto)
Mishandled every respected and feared Family veteran who got out of prison to the point where they were insubordinate and ultimately had to be either put down or sent away (Richie Aprile, Feech, Tony B.)
Allowed a drug addict to live and even reach the upper echelons of his Family, despite said drug addict threatening to kill him publicly and having a rat girlfriend (Chris). After Chris gives up Adriana, he's promoted to Capo.
Tried to boss around New York even though he was just an NJ Don.
Sparked a war with NY which resulted in the death of his Underboss and the mortal wounding of his longtime, wise Consigliere.
Protected Tony B., even to the point where his own soldiers were considering mutiny, out of sentimentality, despite knowing that Tony B. had to die. And not letting Phil handle the murder.
Protected Vito, even to the point where it nearly started a war.
Alienated Hesh, his second best advisor, most low key and intelligent guy and a well respected loan shark.
Lost Barone Sanitation, his Family's main source of legit income.

Under Tony's regime, the Family Lost:
Pussy (competent and longtime loyal soldier, resorted to selling H due to being underpaid and as such was caught and became a rat).
Richie (alienated Richie to the point that Richie wanted him dead, ruled against Richie at every turn, lost an old school, tough, feared Capo)
Feech (Due to paranoia, lost a totally old school, feared and RESPECTED Capo)
Ralphie (The best earner the Family ever had)
Bobby Bacala Sr (Wasted a great, albeit sickly hitman, on a hit that could've been done by anyone else)
Gigi Cestone (Had he not been made Capo, he wouldn't have died from stress)
Silvio (Had Tony not pushed NY to war, Silvio would still be walking)

Yes, Vito and Michael Corleone made mistakes, but they never ran their families into the ground. Paul Castellano was killed, but he died with the Gambino Crime Family as the most powerful in the country. The only other Bosses I can think of who were as bad as Tony are John Gotti and Al Capone.

I mean, even compared to the other Bosses on The Sopranos (Carmine, Sr, who served as Boss for over 30 years and died of natural causes; Jackie, who was loved, respected and served for four years before dying of natural causes; Johnny Sack, who was feared and died of natural cause), Tony sucked as a Don.