Re: use opamp or comparator
The first question is very vague but................in general terms, the failure rate is not related whether it is an opamp or a comparator.
To your second question: You can use an opamp as a comparator, and the web is filled with circuits doing so. But if you are using a generic plain vainilla opamp, it will be slower and the output voltage may not swing all the way up to the rail. My advice is when you have a "compare voltage" function, is to use a proper comparator device.
On the other hand, you CANNOT EVER use a comparator as an opamp.