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There are a number of tiny insects that can ruin buds. You will need to be a real sleuth to pin down which one may be causing buds not to open. Here is an article that runs down the possibilities. Click on each insect for more information on that pest, how to ID it and treat it.
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-flowers-hibiscus-blossoms-falling-off-plant.htm

As long as when you pull it out of the container and the roots fill the container, then you can plant it out. Planting before the roots have fully formed will result in a dead shrub. So as long as the roots fill the container, then it will be safe to transplant.

I noticed that it is only new growth that is affected. To me, this is a sure sign of aphids or thrips. Both damage tender new stems and buds. So get up real close 'cause these are very small insects. Aphids can't fly and can't walk very much. A blast from the hose that knocks them off is usually enough of a treatment. The links below talk about aphids, thrips and other hibiscus pests.
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/pests-on-hibiscus-plants.htm

Stink bugs are a known pest of hibiscus. They don't necessarily smell until they are disturbed or crushed.
https://www.pestworld.org/pest-guide/occasional-invaders/stink-bugs/

Here are some other possibilities:

http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/INVERT/fullerrosebeetle.html

https://www.google.com/search?q=bugs+that+look+like+stink+bugs&tbm=isch&tbo=u&source=univ&sa=X&ved=2ahUKEwiym_rixubcAhUFXCsKHfZzCrAQsAR6BAgFEAE&biw=1391&bih=681

As far as bio-rational control: hibiscus flowers are sensitive so don't use neem oil, horticultural oil or insecticidal soap products. Pyrethrum (chrysanthemum extract) is effective for contact knock down, but has no residual effectiveness as a protective barrier.
I would choose a a pyrethroid, a synthetic version of the pyrethrum chemistry that has knock down power as well as two or three weeks residual effectiveness. It is low toxicity for mammals, but will kill fish and bees and beneficials if you do indescriminate spraying of the entire garden, but if you don't have massive hibiscus plantings and can spot spray your hibiscus buds and flowers, you will do minimal damage to non-target organisms.
https://www.domyown.com/monterey-bt-insecticide-p-2821.html

It's possible that the brown patches are sunburn, but I don't think so. Its not exactly characteristic of sunscald which usually has transitional areas of discoloration surrounding the necrotic tissue. Hibiscus is normally tolerant of full sun, unless under-watered and drought stressed.

A photo of the entire plant showing how much of the plant is affected, and the site conditions would help us to determine what is going on. As well as a description of your soil and water management.

I sounds like a pest or fertilizer problem. Have you done anything different with it than you would? If not then check it for bugs and pests. They may have affected it near the root system.