The biggest problem I had with plastic pressure barrels was infections and beer going off. I had 3. I cleaned, sanitised and bleached the bejesus out of them, tried for many years to get rid of it but just couldn't. I'd brew a batch and put half in the keg and half in the bottles, bottled batch would be fine but keg would always go off after a few weeks, under CO2 pressure. Tried for years to find the problem including multiple posts on this forum but never fixed it, hence my comment above. Add to this the numerous seal / pressure problems and I just lost the will to live with them.
I'm glad some are using them successfully but they just didn't work for me.