Cyufjy

Next year the Keukenhof will be open from 21 March until 19 May 2019. We will be in Amsterdam 27-29 March 2019. How different will the early bloom be from the peak bloom in April? Their website says that the large tulips will not be in bloom:

Flowering at Keukenhof parallels that in nature. The crocuses, daffodils,
hyacinths and early tulips flower at the start of the season. Other flowers
bloom later in the season such as the (larger) tulips. Our pavilions feature
entrancing flower shows from the first to the last day of the season.

Is a 28 March visit still worthwhile or will it not look like the pictures on the website?

When they open the park there will be enough flowers to make it worthwhile. But the opening week there might be more boxes with flowers between the planted flowers and fewer flowers in the ground.

But as much of the available flowers in the ground depends on the winter, which still has to start, it is hard to predict how many flowers will be around.

When the winters are very light, the park is known to open earlier, if the weather is cold in the winter they might even not open on the given date but that is rare.

Whether it will be worth it for you, I can not say.

I know someone from the USA who used to live in Italy, who would always go to Keukenhof in their opening week. (Often while in transit but he did also travel from Italy and back there for it.) He clearly thought it worth it.

On the other hand, you will not see me there, even though it is only about 30 minutes by car for me. I have been there, end of the season and enjoyed it, but once was enough for me.

In short, it will be worth it if you love spring flowers.
Specially if you are near and the travel effort will not be exceptional.

If you will be in the area longer, going later will be better but going early will still be special.