Like anything around the home, greenhouses will require maintenance and, from time to time, some repairs.
Since they are fairly well sealed, water (and therefore mildew) will build up on the walls. Even those with gaps under the roof line and venting fans still retain considerable moisture, and algae, mold and mildew will form on the surfaces.
Both for the sake of the plants, and your own health, that will need to be cleaned off from time to time. Fungi and mold that cling to walls don't just stay there forming an unattractive appearance. They spread, over the walls and through the air onto plants and into your lungs. Not good.
Take a cloth or sponge and wipe the surfaces dry from time to time. If mildew has built up, it's usually sufficient to use a commercial spray to get rid of it. Sometimes you need to work a little harder to get into the cracks with sponge and spray.
Floors, too, can accumulate unwanted compounds. Many greenhouse floors are wood, gravel or even just dirt. While they don't need to be as clean as your house, any of those will require some treatment from time to time. Wood can accumulate slippery-when-wet mold, gravel allows weeds to grow through, and dirt can become mud. Even cement will need to be swept or cleaned periodically.
If you have a greenhouse roof made of certain plastics, you'll find that over the course of a few years it may well darken or yellow. That change reduces the amount of light transmitted and ultimately leads to cracking. It will need to be replaced sooner or later.
Even ordinary glass can yellow over time and become brittle. As it does it transmits less light and is more likely to break from hail or accidental bumping. Though this will take much longer than most plastics, you should be prepared for the possible replacement cost after 7-10 years or so.
New polycarbonate panels are making their way into greenhouse construction. They transmit light well, have very good heat retention properties, and can last for decades. But the seal between panes can require repairs from time to time. Re-caulking is usually all that's needed and can be carried out with a good scraper, a caulking gun and a lot of patience.
Incidentally, condensation within sandwiched polycarbonate panes, and between the panes, walls and other parts can also become covered with algae and mold. Be prepared to clean them, though it won't be required as often as glass.
Ice build up, sharp temperature changes, falling branches and other events can require replacing a pane. A modular construction plan can make that easier and less expensive. Plan or select your design accordingly.
Of course, there's repair and maintenance of watering systems, benches and table tops, and so forth, don't forget. And you thought it would all be just gardening...