There is very little relationship between the people who can properly maintain an aircraft, and the people who can maintain one. I know many people who can do both well, and many others who can only manage one responsibility (and just a few who cannot manage either adequately).
The premise that the owner of an amateur built aircraft could maintain it, and therefore was approved to do so, was based on the idea that that person had built it. Good chance that if someone built an aircraft, and successfully flew it, they can safely maintain it. But, Once that aircraft is sold, the purchaser likely does not have the experience of building that aircraft as the basis for having the skill to maintain it. That does not mean that they can't do a good job, but there's no assurance either, unless they are an experienced maintenance person.
I've frequently heard the attraction of a non-certified aircraft as being the opportunity avoid paying for professional maintenance. This must be the worst safety minded attitude ever. You're already starting with an aircraft which might have been maintained just outside the "norm" of aircraft maintenance (or could be well outside the norm, if the seller was not the builder either). Now the new buyer has to not only meet a safe standard of maintenance, but probably bring the whole aircraft up to that standard as well.
There are many excellently designed and maintained amateur built aircraft out there, and they are a true delight. Indeed, there are very few which are of inadequate design for what they are...
But, economy of purchase and operation is a really poor reason for going the amateur built route. It simply costs money to be safely off earth. If you want to be up there safely, prepare to pay the cost. If you're spending less, you're either flying on borrowed time, or flying an aircraft which is deteriorating as you use it - which I guess is also flying on borrowed time!
Like many certified aircraft, the C150 is very well known and understood. They have their weak points, though not as many as some other types, even some more recent Cessnas. The condition of these aircraft can be very accurately determined, thus the costs to maintain. Non-standard types can be more difficult to cost, and can be equally surprising come maintenance time, no matter who's doing it!
Like everything; in aviation, you get what you pay for!