Gents,
This is all about selecting your most precise weapon and minimising collateral footprint. Everyone's seen the video on BBC and are, for reasons various, baulking at the cost per missile of DM Brimstone. My take is that there aren't very many people on PpRuNe who know what the other considerations around the wider target area were at the time of such attacks against ISIL - these could very well have driven the selection of that weapon so I'm not going to armchair critique the weapon choice and really couldn't give a damn about the cost. Nope.
CRV-7 is a great little weapon; cheap but unguided (i.e. non-precision, which is a bad word in modern conflict), so you've no really sweet moving target capability and you'd need to fire a number on one pass which isn't a good choice when you've a one shot kill option in another weapon. [Engines, granted, if we had an APKWS solution it would be a different story.]
Cannon is good too; lower dispersion, many varieties of bullet to choose from etc. Again, you'd need to fire quite a lot of rounds close-in to the potential threat.
For me, DM Brimstone is an excellent weapon against small, agile/moving targets where you have collateral concerns. We don't have anything else like it on UK Combat Air and even our US cousins are quite envious of its capabilities, proven in Afghanistan on many occasions.