Just my two cents, but I think you're both right. Somewhat.
Imidacloprid is an excellent solution for pest control, assuming you know how it works and use it accordingly. Insecticides are generally classified as contact, translaminar, or systemic. Contact insecticides have to actually make contact with or be consumed by the pest in question. Translaminar insecticides show some movement into plant tissues, and tend to be more residual. Systemic insecticides show a great degree of mobility within plant tissues, and tend to be highly residual. Imidacloprid is a systemic neonicotinoid. Neonicotinoids were initially developed with the intention of reducing pesticide toxicity in humans, but they can be quite harmful if you don't use them correctly.
To maximize effectiveness, Imidacloprid should generally be used as a soil drench, not as a foliar spray. Remember, these things tend to be more mobile from the bottom of the plant to the top, not the other way around. As with any pesticide, it's also crucially important to be careful with your math. To minimize its impact on the environment and human consumers, it should not be used on any plant that has even thought about flowering in the near future. To keep a wide variety of pests at bay at the beginning of the season in pepper seedlings, it's great. Using it on a plant that's anywhere close to flowering is irresponsible. It's devastating to bees, which is the reason the E.U. has banned its use (along with Clothianidin and Thiamethoxam), citing Colony Collapse Disorder (CCD).
There's a learning curve when it comes to insecticides. It really wasn't that long ago that organochlorides like DDT were legal everywhere. Many of the organophosphates (Malathion, Orthene) and carbamates (Sevin) that replaced them are still cheap and easily available. Now we have neonicotinoids to replace those. I 'm just that there's no telling what the data will suggest a decade down the line. Caveat emptor.
To address the original post, aphids are a lot like mice. When you have a mouse problem, there's no reason to use a shotgun. The lye in castille soap is incredibly effective against certain soft-bodied insects like aphids. It should take care of your problem without subjecting your plants (and thereby, you) to the possible effects of systemic insecticides.
I also haven't seen any scientific data suggesting that flushing your substrate with water mitigates the levels of Imidacloprid in plant tissues, particularly in fruit that has already set. In fact, all of the prevailing research I've seen suggests that Imidacloprid and its degradates are highly persistent. One study (cited below) found that plants growing in a field adjacent to a field of corn that was treated with Imidacloprid as seed showed detectable levels of Imidacloprid residues.
Source: Greatti, M., A.G. Sabatini, R. Barbattini, S. Rossi, and A. Stravisi. 2003. Risk of environmental contamination by the active ingredient imidacloprid used for corn seed dressing: Preliminary results. Bull. Insectol. 56(1): 69–72.
TLDR: There's a learning curve when it comes to pesticides. We often don't become fully aware of their effects until they've been in use for quite a while. If you're going to use them, it's important to do your homework and proceed with caution. In this case, if you're going to use Imidacloprid, it's important to proceed with caution.
Again, just my two cents.