It's "safe", but it'll get messy real quick.

Some years ago I build a 496" BB stroker. A number of lifters started collapsing when the engine was held at higher rpms for awhile.
Took a long time before they recovered.
Opened up, checked and cleaned the lifters 2 times and they still kept doing this.
Checked the oil for possible foaming but that didn't happen.
Could have been a engine block issue maybe, but at the time I was getting fed up with it and replaced them all with solid lifters which solved the problem.

More recently when I built my 360 engine I encountered a similar problem with certain lifters not staying primed and started ticking.
One lifter never pumped up and a couple of others collapsed when the engine was held at a higher rpm for awhile.
Again, here too I checked out all the lifters but found nothing wrong while the issue remained.
I modified the lifters to act like solids and the problem was gone.

Reading other people's similar stories and experiences online, it seems almost installing new hydraulic lifters are a crapshoot at best these days.
I'm currently at the point that I will never install hydraulic lifters anymore in an engine build if I have the choice.