I am looking to get a throw lever for a Steiner P4Xi and the only options I see are the Steiner branded one and a polymer one made by MK Machining. I'm tempted to get the MK Machining Throw Lever because it goes for about a third of the cost of the Steiner but I'm concerned how well it will hold up.

If anybody has experience with a MK Machining Polymer Throw Lever I would love to hear about it to help make a more informed decision.

I had a Steiner one (brand manufactured aluminium one to clarify) and it broke. I don't really put my guns through hard use except chucking them in my vehicle without setting them in a case first, now and again they'll fall over from being propped up against something at the range but that small amount of use was enough to break the Steiner one after maybe 6 trips to the range. I had already purchase the Steiner lever direct from Steiner so I decided I wasn't going to spend another $75 for that one just to break again, so I bought the MK poly lever. It is very thick and feels very secure on the optic. So far I've had zero issues with function and consider it more sturdy than the Steiner one. I can pick up the entire rifle by the throw lever extension (probably around 9.5-10 lbs with the loaded pmag). The aluminum on the Steiner would have been too thin for that IMO. Again I've never used it in competitions or anything like that so you may experience different results if you put it through harder use like that or hunting but it's been good to go since July 20 according to Amazon.

eta: clarifying which of my throw levers broke.

Not nearly as elegant as a machined lever, but I've been using these Breakaway nylon coasters as throw levers for years.  A 2 pack is around $20 on Amazon, and I've yet to break one.

^ Good idea.  (S&B....sweet)

Have one for a XTR-1-5, been very pleased.   Having some give is a good thing.  Just ordered another last night so...

For $20 bucks, give it a try..  Especially if your cutting weight.  They ARE NOT pretty....but that’s not in my list of priorities.

I had one, it broke.

I have a breakaway coaster on another rifle, I'm going to go with that for a replacement.

I tried one.  
Plastic threads stripped soon after.
Would not buy again.

I've got a total of 4 MKM throw levers (2 for Nikon P-series, 1 Nikon BF1000, and 1 for a Trijicon AccuPower) and they all have brass thread inserts. None thread the screw directly into plastic. To which plastic threads are you referring?

Are you referring to the MKM lever or breakaway coaster?

This. Context matters. There is a reason we have a quote function.

I assume you like them since you have purchased four of them. Have you had any problems with them? How do they compare to other brands (if you tried  any others)?

I've got  a couple of those in use. As you said, not 'elegant' but sturdy and easy to grab

I've not had any issues with them. I like them because they are ~1/3 the cost of the MGM Switchview levers. Sure the aluminum Switchview levers are probably "nicer", but for what I need a throw lever to do, the MKM levers work fine.

ETA: I did have one issue. I ordered one for a Nikon M223 1-4x and when I received it, it didn't fit, completely different geometry. It most likely would have fit the other M223 models, but not the 1-4x. I reached out to Tyler @ MK Machining and he had me send some photos and measurements to see if they could make one that would work, but the odd geometry on the 1-4x just wouldn't work. They ended up issuing a full refund.

This.  And the Owner of MKM also posts on the hide and said there are fakes on Amazon from china coming in.  Make sure you buy from a trusted source...Not the cheapest you can find

I bought two.  They don’t have plastic threads, they have a brass insert.  The brass insert pulled out on one and I have never been able to keep it tight after that.  The other one has worked fine so far but it is a little crooked.  They are printed and the finish is not great.  I would not buy them again.

No issues with mine, if your Breaking them, I would be surprised....

I have four or five of them I've gotten when they do sales. No issues with any of mine. I've also got some MGM ones, they are nice but IMO not $60 nice. MKM is gtg. The aluminum ones are nice too.

Me too...

Thank You to all who commented, your replies (especially that bit of information about Steiner's aluminum throw lever durability) led me to order a MKM throw lever which might not be elegant but it appears that it will do the job.