Let’s analyze this phrase.
側面 means “dimension”, “aspect”, or “side”, right?
For instance, 経済的側面 means “economic dimension”. Walt Disney’s works were much greater than Apple Computer’s success if you see its economic dimension in the 20th century, in a sense.
The nuance of 〜と言えよう can vary depending on the context, however, it basically means “I could say that 〜”, “you can assume it as 〜”, or “it would be true to conclude that 〜”, ok?
〜的側面が強い is commonly used to point out some feature (a bit) different from what it usually looks or is recognized among people. It can present an opinion from another viewpoint.
Now, take a look at the original sentence.
思想史 is “history of ideas”, the meaning of which is very similar to “history of philosophy”.
The theory ”顕密体制” was originally written with the intention of defining the establishment of some Buddhism groups, however, this author points out that the book(theory) had a significant feature as a history of thought (in the whole history of Japanese religion, perhaps)
In short, the author doesn’t mean to “criticize” it. (At least this short paragraph doesn't look so.) This sentence sounds to me like; “This book is also valuable as a chronology of Japanese religious philosophy” or, “The intention of this book seems to be more being a chronicle of ideas rather than being a dissertation of some specific Buddhism groups.