I use PET plastic fruit juice bottles for small batches (3 Litre - just less than a gallon).
PET is the clear plastic stuff, but the plastic fermenters from the Home Brew shop are usually the white plastic variety - but still food grade.
I would trust the clear PET more than the white plastic, but for primary fermentation vessels (used the first couple of weeks) the white plastic might be OK.
For aging, I make sure the PET bottles are filled right to the top, minimising oxygen exposure. They seem good for at least a year.
Supposedly the plastics let a small amount of oxygen through,
but I haven't noticed appreciable "browning" due to oxidation.
Sanitising can be an issue, because they retain aromas (especially if you do a chilli mead

).
Letting some super strong metabisulfite sit in it for day or two usually helps.
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Another option someone on the Mead Lovers Digest mentioned a year or two ago, was that they use food-grade plastic liners (from the medical industry ??), and then it doesn't matter what sort of container you use.
The liner was cheap and was thrown away after use, IIRC.