This has to be treated more as valuable original source material than as a history. Indeed Napier's history is often quite hard to follow or understand events; and descriptively doesn't tell us too much. For much of this volume I do wonder what were his sources? Where did he get his geographic and historical information outside of his own experience and travel as an officer with the 43rd Regiment? I know he had access to several French sources including Soult's papers, but there is clearly other material which surely is now completely inaccessible to the ordinary reader. In particular Spanish geography is a very important aspect of his historical explanation.
Where Napier is best is with his own experiences - like the end of this volume with the combat on the Coa; but also he is very interesting in his opinions and judgements. Napier also appears pretty sound in his strategic analysis. It would be interesting one day to read his campaign accounts side by side with Oman and Fortescue, but that'll have to be another day.
The volume has good maps albeit not to scale, yet they are difficult to relate to the text at times. Napier's opinions and biases are of course valuable as source material as well as being just very interesting, even if to be taken circumspectly. Soult can do no wrong, he admires Napoleon at least as much as Wellington, but his own government - particularly Perceval and Canning are condemned alongside the Portuguese regency and the Spanish juntas.
In summary, strange to say, Napier would actually be best read today if shorn of the semi-factual historical account and we could simply just read the opinion and analysis, which is the interesting part. Nevertheless there remain a few tidbits of historical note that aren't found in the better 20th century histories of the Peninsular War and best these don't get lost.