Here in the Netherlands the bus companies are required to buy by tender which has led to the purchase of lots of cheap Byd busses.
While they are fast enough and silent they are not that great: there are software problems, heaters don't work, doors won't close and the seats are so bad the (already scarce) drivers are getting back problems.
As a general rule, in transport fleets, unless the old thing is actually basically at the point where it needs to be scrapped right now, the new thing always has more downtime than the old thing, because the old thing has had its kinks ironed out, and the maintainers are used to maintaining it.
Of course, the trouble is, sometimes the new thing is just bad. A commuter/metro train system in my city operates mostly on 40 year old rolling stock. In 2000, some new trainsets were introduced, from two different manufacturers. One model never really worked properly, and was scrapped in 2008; the other survived. The whole thing is now due to be replaced with shiny new trains, starting this year... hopefully these ones work.
Yup (the scrapped Alstom DARTs). But it happens everywhere; for a start we weren't the only users of that problematic Alstom model. You see a lot of old train rolling stock around, but there's a hell of a survivorship bias; a really alarming number of train models last less than a decade in service in any given operator.
You'd _think_ this wouldn't happen, because reliability is absolutely the number one thing that operators care about, and they're usually willing to pay a premium to get it (the alternative being to pay to keep their reliable 40 year old units in service). But the industry's surprisingly bad at delivering it.