A taxi test or 2 is nice for marketing, but does not necessarily prove suitability for operational use.
Even 300 lb is significant in the nose gear. The retraction mechanism may be stressed by it, causing premature failures. One air turnback caused by a hydraulic failure would offset a LOT of tug time...
While it may be less of an impact on a 747 main gear, it is also likely that more/larger units would be required. Electric load would be substantial, and the APU generators may not be able to drive all that plus the standard "house load." Load shedding starts in the galleys, which would not be appreciated by the flight attendants.
Finally, nosewheel mounting would not likely work very well on slippery ramps, i.e., in snowy climates in the winter. Mounting on the main gear would be significantly more complex, since it would also have to withstand brake heat, and prove to NOT interfere with brake operation or heat dissipation.