Poems in any form can be written well or badly. Many of the very complex forms such as the villanelle which rely on complex combinations of meter and rhyme are very difficult to write well; unskilled poets attempting them often struggle so much with the mechanics that they end up writing really horrible poems, choosing words just to fit the metrical scheme rather than for poetic effect. Just because it's a difficult form for an unskilled writer, though, does not mean that it is inherently a bad or trivial form in the hands of a skilled poet like Thomas.
Thomas' poetry in general relies heavily on a Welsh tradition of hypnotic use of melody. None of the syntax in the poetry is wrenched awkwardly to fit the meter, and the almost litany-like quality of the repetitions contributes to the solemnity of the subject. The refrains are used well, with the grand and magnificent tone of the first five stanzas leading to a dramatic twist in the pleading tone of the final stanza, in which we discover that the poet is actually addressing his father. The formality of the form prevents the subject from becoming mawkish, and the form is handled with great skill.