Hi all,
I don't trust rainwater running off the roof due to bird poo etc. One problem which may come about is if you live near any industrial areas where pollution maybe a problem. In these situations, it is usually wise to collect rainwater after a few minutes when it is raining. If it was I, I wouldn't place the water butt under or near the roof.
Pros of rainwater? It is free and won't have a high tds etc, good for top ups and an alternative to RO water.
Regarding test kits, I don't trust them, never used one and never plan to in the future, they are not accurate.