Can mini implants be used to replace missing molars on bottom?

It is rare to see implants fail in general so I would be curious as to why the implants are failing.  Is it periodontal disease, smoking, drinking, systemic metabolic diseases, lack of hygiene?  Mini implants have been shown to have similar success rates as conventional implants as long as they are used in the right situation, but to answer your question, yes mini dental implants can be used for molar replacement.  I would recommend either two smaller (2.0/2.5mm) mini implants to be placed per tooth or one larger diameter (3.0) mini implant per tooth for molars, but I would be certain I know why the past conventional implants failed because I wouldn't want to have the same things happen.  
As far as price goes, generally speaking, minis are about half the cost of the conventional implant treatment so it is definitely worth pursuing.   Make sure you find someone who has experience with mini dental implants as not every dentist/surgeon places them.