Without sounding trite, the internal slave is a "problem" only when it fails. These release bearings can last a very long time, and while working properly, they are smooth and efficient. When they fail, the leaking hydraulic fluid (DOT 3 brake fluid) can quickly ruin a clutch disk.
For the most part, we only hear the negative side in this story. When the clutch gets bathed in brake fluid and slips, that's a challenging and costly issue. Servicing/replacing the release bearing requires detaching the driveshafts and removing the transfer case and transmission. The external slave eliminates this level of work if the slave cylinder leaks or fails.
Hydraulic clutch release bearings have been widely used. Jeep was neither unique nor a minority when AMC introduced the use of the hydraulic release bearing in the XJ Cherokee and the YJ Wrangler. As for service concerns, from my vantage, the release bearing should always be replaced during a clutch disk and cover change out.
If you keep the OE hydraulic release bearing, watch the bearing for any signs of seepage, much like you watch for a rear main seal leak or other leaks. The hydraulic fluid, being DOT 3 brake fluid, is easy to spot. If the clutch gets "spongy" or air suddenly begins to enter the hydraulic cylinder (requiring bleeding), replace the release bearing before the seal ruptures completely.
Watch the master cylinder for fluid loss. Fluid height fluctuation from clutch disk wear is normal. These master cylinders can fail or leak internally, causing a soft, spongy or "falling" pedal. Hydraulic master cylinder problems prevail with either an external slave or internal hydraulic release bearing.
As for the external slave, these are not an end-all problem solver either. The slaves leak and bleed down internally with wear. Again, the master cylinder is no better than the master cylinder with a hydraulic release bearing.
I have built Jeep 4x4s from scratch and chose the external slave over the hydraulic release bearing. However, scrapping the OE internal hydraulic release bearing design has never been a "must" on purely stock Jeep YJ Wrangler or XJ Cherokee models.
Most YJ Wranglers and XJ Cherokees only require a hydraulic release bearing change during clutch disk and cover renewal. If I were pressed to say how long the hydraulic release bearing lasts, a lot depends on how many times the bearing is operated. Driven in city traffic or constantly on crawl-pace trails, the hydraulic release bearing does wear out faster.
Your call here, Diamond Jr...My two-cents worth!