.would the crest be tough enough for a 140mm travel hardtail
I don’t think they would from my experience but it would depend on how heavy and how smooth a rider you are.
Really you should be thinking of either the Flow or the Arch, as the Arch is one down in terms of weight, strength and width from the Flow, whereas the Crest is quite a big jump down in those terms.
I’ve run both Flow and Arch on both a hard tail and an FS bike. I’m 100kg, fairly agro and more of ‘balls out regardless’ than a ‘smooth and refined’ rider.
The Flows are super tough and can be set up pretty light with the NoTubes tubeless kit but as someone has already pointed out, they are harder to set up without rim strips, whereas the Arch rims are easier to run tubeless with just yellow tape.
The weight saving for just using yellow tape rather than rim strips is 50g per wheel. So not insignificant considering it’s rotating weight.
I ran the Archs on my hard tail for three rides before I more or less destroyed the rear. I don’t think the rim is stiff enough to resist twisting and therefore puts a big strain on the spokes causing them to unscrew. My rear wheel more or less came apart after three rides over the very rocky trails of the Peak District.
The Flows on the other hand are utterly bomb proof and very stiff and I’ve never had any issues with them, despite running them on some pretty rough DH courses, including a three day stint in Chamonix last year.
For a well used hard tail, the Flows would more likely be the safer choice than the Arch and 100% the better choice than the Crest.