Slides don't "tend to under-expose". They are just unforgiving to exposure mistakes. If you have a badly exposed slide, it just means that you or your camera has an error in exposure. Just have to learn from mistakes and be more careful. Otherwise, slides are pretty satisfying to use.
Actually dun think Konota and Ruby can be considered pro-labs. Some of their processing can be sub-standards sometimes. RGB is a commercial lab which does a lot of processing for industry use, but it's a bit ex. Alternative is you can get it sent to Fuji lab at Ubi thru your regular Fuji shop.
Actually RGB is the cheapest for Slide processing, only $5. However, they can sometimes return your slides with some irritating scratches on them. Their main advantage is the fast 2hr turn-around time.