I have had no problems with avast apart from it killed a computer by trying to do too much.
One some computers here it is fine
One I only just managed to get it back back to booting windows after a week.
No Safemode, no lastknowngood, no system restore.
I looks like the additional updates module may have been the issue, so problems could be varied and difficult to trace to the cause.
Software will have problems, the difference is if problems get fixed, or the damage is limited.
Users don't want to have to be experts in a constantly changing utility service. They have better things to do so they pay for products so they don't have to. They just want it to work.
Saying it is 'fine for me' doesn't help those with problems when an antivirus update causes as many problems as a virus and leaves a computer useless as far as the average user.
It makes people wary of any updates.
Do you feel lucky? can you do without this computer for days or a week until someone can fix it?