Answer: Off hand I can't think of any negative effects of using compost, unless you have used diseased or otherwise inappropriate materials in the pile and it has not heated up sufficiently to render it safe to use. (Many home piles do not heat up to ideal temperatures, so this is a possibility.) I suppose some plants might grow overly lush if you lavishly used excessive amounts of a compost that is very rich in nitrogen, however it is difficult to make compost in such quantity that you would ever have "too much" of it. The only caution I can think of would be to take care if using fresh stable manure or a large proportion of fresh grass clippings so that you do not "burn" any plants by accident.
In my experience partly finished compost is an excellent mulch, I have also used it as a soil amendment. In fact, some gardeners "sheet compost" by spreading the ingredients over the ground rather than putting it in a pile at all. Any time is a good time to add compost -- and it is good for all kinds of plants.
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