Job hunt problems

I have been looking for full-time work for a couple months now. My question is, how bad is it to accept a job offer while a more desirable job is "pending". Like the old saying goes... a bird in the hand is worth 2 in the bush, right? I currently work PRN but need to get a full-time gig STAT.

For context job A has offered me a position. I have been pursuing job B and they have showed interest, but it's for a company that is known for dragging thier feet (it can take 8 to 12 weeks for onboarding). 

Job A pays less but decent, offers less hours and does not have a great benefit package. I do think I will like the "work", as I have worked in an ASC before (different specialty) And I can start like next week. It also appears to offer more schedule flexibility. 

Job B will probably be 8k-10k per year more due to higher pay and more hours. They also offer no cost family health, dental and vision insurance, which is a huge benefit. It's union.  Job B is in the specialty clinic for a specialty I currently work in, so I think I can adapt well . I don't particularly like "the work" but it's in my comfort zone. 

I am just seriously considering starting job A and possibly leaving if I am selected for job B. On the other hand, just start Job A and stop pursuing job B. Please tell me the pros and cons of this that I may not be considering? It seems like 5 Years ago it would be detrimental to quit a job so fast but these days,  people are so unreliable and change jobs like underwear and it's no big deal. Open to any feedback.