Having previously done the CRI course, some time prior to doing the full FI course, I feel able to comment on the relative worth of the 2 qualifications.
In my view, the CRI test, as Genghis points out, is a bar quite high enough for the work that will be done subsequently. It's NOT about ab initio instruction, it's about passing on enhanced skills or assessing current PPL skills and helping people whose skill levels might have drifted off. It's also about helping people gain knowledge of a particular aircraft type - the LAA coaching scheme requires people to have the CRI qualification. The CRI ground school fully covers all the teaching and learning skills also required of a FI. I found the test a challenge and felt pleased with myself that I had attained the necessary standard.
The FI is all about helping people take their first aviation steps and therefore covers in great detail all the initial exercises so that the student can pass the PPL (or LAPL) skills test. The FI initial flight test is not actually a huge step up from the CRI flight test, or mine wasn't, anyway - maybe my CRI test was a bit tougher than other people have enjoyed.
I'd be more interested in comments from people who have actually done both courses and tests, rather than those expressing impressions from a distance.
TOO