Those are very broad questions. (Hey Russ this is an actual English character set I can read as I type).
First, there are a number of CTLA methods, not just one. If we talk in the singular we should talk of the CTLA Guide... the Guide for Plant Apprasial 9th Ed.
There are a numnber of ways that it can be improved. Look at the archives here under the topic of 9th Ed. Generally where the Guide needs improvement is in better articulating why things might or might not be done in particular ways. There could be better consistency with underly theory and from method to method.
We must understand that while the Guide lays out the three traditional approaches to value (Depreciated Replacement Cost (DRC), Market or Sales Comparable and Income) the CTLA methods are all DRC approaches. The DRC approach is well suited to appriasing landscape trees which are not market goods and for which there are not market data or monetary income data. The DRC approach is also laid out, as it fits the Trunk Formula Method, in my article which is linked here on the Tree-Tech web site. That article was written in the context of the Uniform Standards Of Professional Appraial Practice (USPAP) and the standards of the Apprrraisal Institute in the US and Canada. I've just finished teaching a two day course in England and DRC for trees is also strongly supported by the Royal Institution of Chartered Surveyors (RICS) Red Book.
DRC is the strength of CTLA methods. But in some jurisdictions it is not applicable. The IRS does not accept replacement cost as a measure of damages. Some states do not admit replacement cost in court either.
But for practice in the US CTLA is the best, most comprehensive and most widely accepted guide. There are a number of methods in othert countries. May are laid out in an article by Gary Watson in a recent Journal of Arboriculture.
So you really need to do some homework on the CTLA methods to understand both strengths and weaknesses.