I've been transitioning to a recumbent bike myself after years of riding a standard upright bike. After landing on my head in a bad crash last year I feel safer going feet first, just a subjective choice thing. I have no physical limitations.
Recumbents are different machines and work your body slightly differently than an upright. There are differences in their performance compared to an upright - they generally (but not always) climb hills slower, but they go faster on flats. They are also much easier on your body because of the way your body is supported.
As far as exercising effectiveness, recumbents work you just as hard but avoid problems like wrist, neck, and back pain, as well as various pelvic issues.
Recumbent bicycles are difficult to learn to balance. They usually are like learning to ride all over again. Trikes are simple. You'd have to go off the road to turn one over.
But if you are talking about something held in a trainer indoors, much of the same applies. In my recovery I rode one of my uprights in a trainer 40 minutes a day. That time goes by really slowly. Others have reported using their trikes in trainers and having a much better time.
Comfort on an upright certainly is possible (I put many thousands of miles on mine) but it takes some effort and you have to know what you are doing. Comfort comes with a recumbent. It's actually difficult to get uncomfortable in them.