Not trying to take sides but I have seen the work of guys that have been in the Tree trimming business, Plumbers, Electricians, Pressure Washing Contractors, Landscapers, etc..... that even though they have 15 to 20 years, that just means that they have been in the business that long and a lot of these guys still don't know what they are doing.
The guys with the licenses (plumbers and electricians), all they have to show is that they have been working with a company for so many years then pass a test. They could have been helpers and do good on tests, that does not mean they could wire up a house or building to proper code, just that they can hold down a job and do what they are told most of the time. The plumber.....just means basically the same, they hold down a job, can pass a test and do what they are told, does not mean that they can run pipes through the house to code, pressure test them, make sure that there are no leaks in the hidden areas, etc......
There are complaints all the time about guys that have a lot of time in the field but time does not really mean anything these days as a lot of those guys just fell through the cracks and are finally getting caught doing shoddy work and people are getting fed up and are finally doing things about it like taking them to court, filing on their insurance, calling the media, etc....
Time really does not mean that much, what you can do, what you can prove (videos of work being done properly, pictures showing before and after the job, testimonials of reputable people or people in the public eye, etc....) and no filing on your insurance and getting taken to court and no businesses bad-talking you are a good indicator that you are good at what you do and are a good contractor.
The certification is both a good idea and bad.
It is good because it shows that you can take and pass a test, come up with the money for the class, learn the current ways of doing things and the way that it is supposed to be done, etc...
It is bad because there is no guarantee that you will always do the job right just because you have the certificate of attending the class.
Every year restaurants burn to the ground and sometimes if not a lot of times it is because of the vents not been cleaned properly and that can come back to the owner not spending the money for a reputable contractor to do the job right or it might have been a contractor with a good reputation that charges appropriately but does half-assed work and might end up getting sued and might not even have insurance.
Just a little rambling here about the good and bad about being certified, just my opinion, not the opinion of PWI or NCE.