I have seen warnings for buses in most guidebooks and here on VT. I must say that I always visit Plitvice on a daytrip and usually it is during the summer.
Because there is no real bus station here but just bus stops along the road (next to both entrances) you should simply follow few rules in order not to get stuck by the road untill late in the evening:
1. Leave early. Catch an early morning bus, and even if you take the longest route and take your time for a lunch it is easier to wait for a bus at 5 pm than at 10pm. Even if you have to wait for a bus for the whole hour in the first case you'll board it at 6 pm which is OK.
2. Avoid Friday afternoons if you are continuing from Plitvice in direction of the coast. And avoid Sunday afternoons in the opposite direction when all the buses returning to Zagreb tend to be full and will NOT stop at Plitvice!
3. Do NOT buy a return ticket! This way you will have to use the same bus company on your return trip, and that is very limiting. Buying two separate tickets is more expensive, but gives you more choices!
If you follow these rules you should be OK and don't have to worry about returning back or continuing your trip.
There's no danger whatsoever in Plitvice Park.
It's how to reach there & how to get to your next destination !
There's a bus stop on the main road just next to the park. There are many buses going in each direction, either to Zagreb or to Dalmatian Coast (if you want to reach there).
One warning : No matter what the time tables say, you might not find the right bus to reach your next destination especially along the Dalmatian Coast !
My advice is to FLAG EACH & EVERY BUS THAT IS GOING TO YOUR DIRECTION & ASK THE DRIVER.
You will thank me later about this. Some drivers will take you on board & drop you off at a station where you could find/take your next bus to your planned destination.
If you are lucky, yes of course you can take the bus at the bus stop I mentioned above. I stopped many buses to Split but not to Zadar.
I had to wait like 2 hours (at the bus stop) to get back to Zadar where I put my luggage.
A bus bounded for Split took me for a free ride & dropped me off at this place in Korenica (photo on the left). It's a restaurant by the road where many buses stopped for a break time between journeys.
I was lucky there because there were 2 buses to Zadar at that moment !
Plitvice-Split : 3.5 to 4 hours
Plitvice-Zadar : 4 hours.