If they did it to the IDF, I doubt many would have an issue with it. Military is always a valid target in war.
If it was done to random civilians, then most would.
It all hinges on just how targeted the attack is, and how much you believe Mossad's claims that it was all Hezbollah. We've seen their claims on bombing Hamas militants, and I doubt everyone in those 8 floor residential buildings were all Hamas members.
If they had snuck explosives into cell phones of IDF soldiers and then detonated them in mass while the soldiers were at home with their families it would not be acceptable.
I totally agree that this attack was indiscriminate. I think part of the point was the psychological impact of being attacked that way. How could you ever feel safe again after knowing anything around you could be a small explosive device.
It is a huge escalation that will undoubtedly created another huge wave of terrorism. Israel has always been its own worst enemy.