For us, using St Pancreas to catch Eurostar is a pain in the arse; Waterloo was sooo much easier.
The fast direct train to Amsterdam was a piece of cake. Check in, customs, border control, simples. Get to Amsterdam and just walk out, no problem.
The only issue was the train was not as comfortable as it cold have been and was cold.
For the return we had to change at Brussels, it was a nightmare. Amsterdam to Brussels was about 10 minutes late (warm, comfy train). The down escalator at Brussels was not working, we had to walk
down it. Caused many people lots of problems; elderly with luggage, people with big heavy bags. The route from one platfor to the next was not well signposted. The security staff, customs, border
officials were very officious to say the least. Person in front of me went through security (her bags and loose stuff had already gone through the scanners) and got sent right to the back of the queue
again, because she was wearing a watch

After we had gone through security, there were last call announcements for the London train. I suspect some people were still trying to find where to go at the point
and with the queues long and slow at every point of the transaction, I guess many people missed the train. There were more than a few empty seats on the last leg.
Fancy a nibble or drink on arriving at Amsterdam; this was good
Home
direct access to platforms, one of the best station caffs in Europe.