I haven't done all my old ones yet. I simply bought a scanner capable of 9600dpi, not a dedicated slide converter. What I've seen of the dedicated slide converters is that they're cheap and low quality.
My eyes can't detect any flaws in the color representation but my color vision isn't the best. If you want to be sure it's right get an IT8 target (you must buy one, don't consider any DIY approach!), good scanning software will have a calibration option where you scan the target and it prepares a calibration file for the scanner. I've also seen a similar approach for calibrating printers--it prints a pattern, you then scan that pattern with a calibrated scanner and it produces a calibration file to ensure correct printing.
The tedious part is making sure everything is aligned before you scan. If you outsource it I would worry about people using software to correct the skew--that costs you image quality.
I've not been happy with the outcome but that's because I've seen how much the old shots have deteriorated (most of what I've scanned is 30+ years old), not because of anything I did wrong.