How accurate is Google Play installs measure?
It doesn't take uninstalls into account. It is very slow to update and based on the all-time number of individual Google accounts that installed it, NOT the devices. 

That's all based on what I experienced, from the Play Console combined with Store

Possible values include:

1+

5+

(Maybe some more)

100+

500+

1k+

5k+(?)

10k+

50k+

100k+

(Maybe some more)

1m+

(Maybe some more)

1b+