I think the safest pilots are the ones that have the right attitude, regardless of their qualifications.
I have met and heard of countless pilots who did an IMC rating a while ago, and think of it as a full instrument rating...ie. they are quite happy to go flying when the weather is rubbish, knowing they will be flying on instruments. These same pilots, most of the time, seem to think that pre-flight planning is for wimps. They are usually the ones that we see in the accident reports, losing control and/or flying into granite.
Any pilot who fully recognises the limitations of their skill, currency and qualifications is a good pilot.
I hold a full IR and have done for many years. I've got thousands of hours flying on instruments. However, if I was going to hire an aircraft I am not that familiar with, or haven't got loads of currency in, I would raise my own limits - to me, that would mean not flying in bad IR weather until I am full current (ie. flying in instrument conditions every day), and increasing landing minimas to what I am happy with, giving myself a good amount of safety margin.
I really don't understand the people who say "well I have an IMC rating, I've flown 3 approaches in the last year so I am going to take off (in crap weather), fly to xyz and I can use my GPS to find the airfield and if I need to I can fly the VOR approach coz I have an IMC rating so I know what I am doing".
These people scare me.
No-one should ever consider an IMC rating as a proper instrument rating. If you have one, then keep it current (I mean, really current) or forget you have it. You shouldn't get yourself into the point of needing to use it but if you do then at least you should be able to get yourself out of trouble, land, and understand how you can avoid the situation the next time.
15 hours is not enough. Imagine if someone could do 15 hours and get a PPL. Yes, they could manage to fly an aircraft. They will probably have already been flying solo. They might think they can fly. So why do the extra hours? It is a similar situation.
PS. I am not trying to say all IMC holders are rubbish because there are some that use the rating wisely and are very stringent on themselves - they ensure they keep it really current and treat it with respect. That is great.