I use metal containers for part of my garden. The galvanized metal tubs work well, can be easily drilled for drainage, and the metal retains heat in cooler weather.
In the Chicago area, I have harvested tomatoes in November from my metal containers. I even use old milk cans.

I don't have a lot of space in the backyard and it works as double-duty, functional and pleasing to the eye.

I paint my galvanized containers as well, but only the outside.

Replacing the ones I did 15 years ago.

Sure, they corroded, but it took many years. And the wood, untreated, grayed out over time. But it’s all natural. And nothing lasts forever.

I’ve thought about painting or sealing, but just couldn’t bring myself to do it. Even sealed, the treated boards are going to be exposed to the elements and will probably fade anyway.

For smaller beds that might be protected in some way, or brought in in the winter, it could work.

Have fun with it.

go for it.

I use black plastic 5 gallon grow bags ( inexpensive, last for many years).

I just need the decorative pots in place to hide the functional part of the garden behind them.

Even with some holes drilled in the bottom, your roots will drown.

Can't beat wood. Paint or stain the outside. I'm lucky that Mr. Bayard brings these big wooden crates home from work, 3' x 4'. They get supplies shipped in from the mother country. They are hinged on the corners, open top and bottom. They stack as high as you want them, and can fold flat. That could probably be replicated.

The leaves of my rosemary always look dried out, no matter how much water I provide. I gave it a hard cutback hoping that the new growth will look more resilient.

I grow a few hundred tomato, pepper and eggplants in them yearly, for decades. Never lost a plant yet.

It should only be in the outside though. If it is constantly moist with wet dirt against it, it will likely peel off.

So rainfall will fall on the lip and into the container. That lip is what concerns me. It wouldn't have looked right if I hadn't have painted it.

Think of a house. Exterior acrylic paint, holds up to all the elements, but if it is has moist soil against it all the time, there will be issues. As long as you keep the paint above the soil line, you should not have a problem. You can use a layer of mulch to hide the bottom edge if needed.

There are primers for plastic products. Or at the very least, scuff the surface with fine sandpaper before painting.